Reading this thread makes me realise just how much 'at home' I feel in the States. Most of these differences now seem totally normal to me. I <i>often</i> find myself asking for 'the check' at restaurants in the UK. Don't think these have been mentioned yet:
Jelly - Jam;
Liquor - Spirits;
Liquor store - Off-licence;
Entree - Main course;
Lines - queues (another one which I find myself using at home - 'stand in line', rather than 'queue up');
Sneakers - trainers;
Downtown - City Centre.
I've completely exchanged the British response of "Fine" to the question "How are you?" for the American "Good". I've tried to stop myself, but it just seems to come naturally. I also find myself asking "How are you doing?" (although not in Joey style!). One thing I haven't cottoned on to, though, is "Excuse me", rather than "I'm sorry" if I accidentally bump into someone, for example.