Sometimes I get free accommodation (recently that included a stay at the Intercontinental, two meal vouchers, transfers, and a city tour when flying economy class on LAN from AKL-BOG with a stop in SCL) when in transit involuntarily (i.e. there is no flight leaving on the same day to my final destination, and sometimes I don't (e.g. when flying business class on SQ on a RTW).

Are there any rules that determine this? Is there ever a situation where you are entitled to it?

Most situations causing a delay are beyond the airline's control (this is called force majeure) and you are not entitled to accommodations or compensation. Some situations where you are entitled to accommodations are shown in the airline's Rule 240 (varies from one airline to another).

It is common to get accommodations if a flight is diverted or makes an unscheduled stop for reasons other than weather. Delays caused by holding the plane for another passenger or due to another unruly passenger, are usually compensable.

Sometimes airlines give accommodations, usually only to first class travelers, when they don't have to.

You should be able to find the airline's rules on its website although navigating to them is probably not easy.

Even if you are not entitled to accommodations, you can usually get a discount "distressed traveler rate" for a hotel stay just by asking an airline gate agent or customer service rep.

Travel tips:
http://members.aol.com/ajaynejr/travel.htm

You should be entitled to compensation for everything but weather. There are new EU laws (whose application will start in Feb) that stipulate exact compensation levels.

When I flew in Econ to New Zealand MAN - AKL with Malaysian I had a 12hour layover in Kuala Lumpur and they provided me with a free day room. It was great, landed at 8.15am, showered saw the sights in KL, had a nap, showered again and was off to NZ at 9.00pm

This was four years ago however!

Although the title is generic, I think the OP is not talking about delays, but rather about an involuntary overnight transit (travelling A to B to C, when a lack of scheduled flights between B and C forces an overnight layover at B). This is most likely known at the time of booking.

I've only had to use this once, on a flight in business on AI where I was told by the TA who issued my ticket to go to the transit desk to collect my hotel and meal vouchers. But I would also like to get an answer to the OP's questions below.

I thionk you're right, the OP looking at cases where schedule requires an overnight (or all day) stop. I too am curious about this as this is fairly common occurrence heading to/from NZ through Asia.

I don't know about now but a few years back, NH's flights to Hawaii (did this) and the mainland USA (I think as well) required an overnight connection in Tokyo or Osaka (can't remember) and they offered a free night at the nearby ANA Hotels...

Oh and TK does as well if you have to stay overnight in IST due to routing requirements. But you had to pay for the Turkish visa yourself.

That's correct, hence the word 'transit' in the title. Sometimes airlines arrange accommodation, sometimes they don't - I don't understand why a failry cheap LA flight gives me this but a RTW Raffles class trip on SQ doesn't.

Emirates schedules seem to very frequently leave you in Dubai for 4-20 hours if travelling from Western Europe or US to the Gulf and back. Many flights are every other day, so they seem to have a very standard policy of including rooms in the price of the ticket.

Depending on layover and class of service you might get 4 hours in the terminal hotel, or a very nice room at a nice hotel off site. (Been a while... I can picture the lobby, but not the name of the hotel...)

They even have a very efficient set up of cars to take you to the hotel.

Sometimes other routings are more direct, but I find this a very civilized way to travel and if you can spare the time, you arrive much more rested.

I had a talk with a Turkish Airlines (AA partner?) representative a while ago and he was mentioning they pay accomodation in Istanbul for J passengers if transit times are "long". Defining that is another matter...

Edit: Sorry, was already mentioned before

I would have to say it's inconsistent. Lots of flights via HKG from YVR to India include the hotel. I have had a free night from HKG to SYD via KL on Malaysian but my flights YVR to GIG via LAX on AC and RG don't include a free hotel room.

On the OT, I have found that even with force majeur the rules can be different. I was flying ORD to YEG via MSP by NW. Our flight was delayed take off in ORD for bad weather. By the time I got into MSP I had missed my connection. Everyone else was told to fend for themselves. I was told that because I was an international flight I could get a hotel (Holiday Inn) and meal vouchers ^ .