Originally Posted by goplaces
I've never transited SFO as it's my own airport. Will you go through customs without claiming and rechecking baggage? If so, you could probably get back to the airport an hour before departure and you'll be fine, since you'll already have a boarding pass. If re-checking bags, I'd personally allow a little longer, but that's just me.
You should be able to check your baggage through to LHR when you check in at SYD, and then you pick up your luggage after immigration but before customs, and then drop it off at the counter just past customs, so all of that will be taken care of before you leave the airport.
I've never had any trouble with SFO international security -- I think getting back to SFO an hour before your flight will be plenty of time...
Definitely get out of the airport when you're here!
In fact, even if your friend ditches you, you should hop on BART and head into SF anyway (it's a quick hop-skip-and-jump (assuming you've dropped off your luggage; otherwise the skipping is a bit more difficult) from the International terminal)...
Dan