It has always struck me that it would be useful if somebody would publish an American/British British/American dictionary or phrase book. This would be very useful for the tourist who is crossing the pond, and could avoid some embarrassment.

Several terms are ambiguous, with different meanings in the two countries. For example in a restaurant, In England, you can use a cheque to pay the bill, whereas in America, you can use a bill to pay the check.

Example of an embarrassed Brit in the US

Sitting at the bar, pencil in hand, struggling with the New York Times crossword, the Brit yells "Hi there, anyone got a rubber they can lend me?", and wonders why he is getting many odd looks.

Example of an embarrassed American in the UK

An American girl was at a party in London, but was suffering from some discomfort in the tight pair of jeans she was wearing, said "Geez, I got this terrible itch in my fanny." and wondered why the conversations all around her had stopped.
Here is a list of useful translations Anyone with more terms to add, please /msg me -- ponder

Update: I've found the following comprehensive lists on-line, though I don't agree with some of the Brit entries:

1/3/2005: Gorgonzola has suggested some updates which are included.