From the Department of State:

"dual nationals must enter the United States with a valid U.S. passport"

Ding Ding Ding, we have a winner!!

I too am always amazed by the arguments from these guys, especially when they're "setting you straight", the whole time not actually answering a question.

I think a very simple rule of thumb is, if you have multiple valid passports, use the one that's required, or if not required, the easiest, at your next boarder crossing. Each step in your travel - checking in and getting your boarding passes, then possibly exit immigration, then entry immigration, then possibly another exit immigration, then another entry immigration - should be considered independent of every other step.

And why, when someone asks a pretty straightforward question, does it always devolve to "why are you a citizen, how are you a citizen, can you vote, why can't you vote, what type of citizen are you, blah blah blah". This is a travel blog, where people either ask questions seeking answers, post information they think others would like to know, or they post tales and tribulations for advice or a sympathetic ear. The amount of dicta given and asked for is simply stunning.

The OP has two passports. It's not important how he got them, he has them. The question is, which one to use in a given transaction on an international trip where he's crossing multiple boarders to get from originating point to final destination. Just answer those questions as succinctly and accurately as possible.

As a dual citizen, I would also confirm that travelling with two passports is a straightforward process. As long as all your countries of citizenship allow you to have other citizenships, travelling is a bliss. The only rule of thumb here is not to enter the country of your citizenship on the "foreign" passport.

Er, Indians anywhere don't need anything special to visit India.

US (or any non-Indian) Citizens of Indian origin however require a valid visa to enter India.

And, no, even Indians residing aborad can not vote in Indian elections.

That had a lot to do with media ownership laws in the United States, coupled with the fact that Australia did not allow dual citizenship when Rupert naturalized to become an American Citien.

Since you didn't provide any good link, I looked it up. They have what they call OCI, which is a status with limited rights. It does put you ahead of those who are not OCI. And I know for a fact the parliament was considering voting rights. But evidently some political groups blocked that. And that part of the news story I missed. But the source I consulted says that the Indian constitution does not allow dual citizenship. Still I know some countries do. And according to my Canadian niece, British Commonwealth citizens have advantages all thru the commonwealth countries. To immigrate from the US is far more difficult than between Hong Kong and Canada.

Yes. Despite the misnomer of terming it a "citizenship" it is infact a life long, multiple entry, permanent resident visa. (And a 'visa' is all it is because you need to present your foreign passport to enter India).

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Granting of voting rights would require a constitutional ammendment to change the clause which currently leads to an Indian citizen losing their Indian citizenship as soon as they have acquired the citizenship of another country.

This also expands to Consular assistance where Canadian and Australian Citizens can seek consular assistance from each other's embassies where their own embassy is absent. If neither Canadian nor Australian Embassy is present in a country, British Embassy is then responsible to extend assistance to citizens of Australia and Canada (amongst other Commonwealth nations). Singapore and Tanzania have opted not to recieve consular assistance from United Kingdom. Also, the British Embassy (rather, the High Commission) does not extend consular assistance to other commonwealth citizens in a commonwealth country.

Here's an interesting article expanding on that. http://en.wikipedia.org/wiki/Commonwealth_citizen

However, these days, when it comes to immigration, I doubt commonwealth countries treat citizens of any other commonwealth countries any differnt than citizens of non-commonwealth countries. The same immigration rules apply.

So you doubt the veracity of the article you gave the link for?

OK, so here's another one:

http://en.wikipedia.org/wiki/Canadians_of_convenience

OCIs are not actual citizens of India and they cannot vote in every election in India. The Indian Constitution bans dual citizenship to citizens of India and that's been the case for over 50 years. More fiction revealed as fiction.

Going back to the original question. This could have some issues - even though the following is the correct way to do things:

At LHR, you would have to advise the check-in staffer that you don't intend to enter Australia using your US passport, but instead your UK passport. They may be only able to issue your boarding pass to LAX and/or only be able to check you in LHR-LAX.

At LAX, you would have to then find a Qantas/V Australia (Whoever you are flying with) staffer and get them to update the booking with your UK passport and check you in/issue the boarding pass.

Check-in LAX-BNE (should / will) fail if checked in with the U.S passport as that passport will fail the automated Australian immigration checks (due to no visa)

Very interesting, thank you. ^

Except for the old "working-holiday" visas which allow(ed) *some* Commonwealth citizens to spend two years in the UK working without restriction.

It's not exactly immigration, but on a side note the UK allows (some, not all?) resident Commonwealth citizens to vote in all elections cf. EU citizens who can vote in local/Euro elections and non-EU, non-Commonwealth citizens who can vote in Dancing With The Stars.

GUWonder is 100% right. Try entering India as an Indian with US passport without a single/multiple entry visa or life long visa termed OCI. OCI is to be applied for and pasted in your passport and you need to carry the OCI card to be produced on demand.
US citizens CANNOT vote in Indian elections as India DOES NOT allow dual nationality.
I am a British National with an OCI card and enter India over 10 times a year. I am not even allowed to have a voter's identity card unless I am a full fledged Indian citizen
The only exceptions are INDIAN PASSPORT holders who live in USA on any US immigration Visa or Green card and enter India on an Indian passport. They can vote in India as they are Indian citizens living abroad.

In addition UK allows commonwealth citizens to claim free health care, children's benefit and social security benefits such as housing and unemployment after you stay in Britain longer than 6 months.

Also, to the OP: no, there is no single exhaustive register or database of people who are US citizens because there has never been a comprehensive tie-up of domestic birth certificate databases, and there are people who are US citizens from birth who are not born in the US. There may, obviously, be a single exhaustive register of everyone who has a valid US passport by now.

Ha ... isn't it great to carry additional passport(s) from (an) other country(s) ... it usually saves a lot of visa-fees and other hassles .. :D