I'm a native American English speaker who's emigrated to the UK, and time-telling in the two different countries occasionally gets tricky. "Half ten" has already been mentioned, but try this on for size:
American: "quarter after (ten)"
British: "quarter past (ten)"
American: "quarter of (eleven)"
British: "quarter to (eleven)"
British: "half ten", "half past (ten)"
"Quarter after" my Brit friends have no trouble understanding (although one of them once did say, "What a curious way to tell the time!" He was a bit camp, though.). "Quarter of" they find completely incomprehensible ("....sorry, I'm too tired today to deal with American time.") and I generally have to "correct" myself to British usage.
So here's a question: anybody know where these differences come from, and whether they're recent or not? And how did we end up with the preposition of in the States? I mean, isn't quarter of ten, like.... 2:30?
EDIT: I very stupidly listed the pronunciation of 10:45 in British and American English as 'quarter of (ten)' and 'quarter to (ten)'. It is, of course, 'quarter of (eleven)' and 'quarter to (eleven)'. It's fixed now.
EDIT 2: My point with 10:30 was more that in American English there's no alternative to 'ten-thirty'; we never say 'half past ten', for example.
EDIT 3: Wow, I didn't realize this was such a regional thing. I'm from Southeast Pennsylvania, and I'm kind of used to thinking of us mid-Atlantickers as not having any kind of regional... well, anything, really. Didn't realize there was so much variety in the States. Learn something new every day!