I've been looking into the same thing and here's what I got.....
An FAA license is good for any N registered aircraft, regardless of it's location. So, if British Airways ever got to their senses and bought a N-reg 747 I'd be fighting you for a seat....problem is, they keep on re-registering the bloody things to a G-reg and screwing us over.
Seriously for a sec, you CAN convert your licenses to the JAA format fairly easily. You need to take 14 written tests and 3 flight tests, although I believe they may waive some of that if you have 500 hours flight time in a aircraft certified for two pilots (no, CFI time doesn't count....sucks huh?).
You can take the written test prep online, and then the real exams either in the UK, or at a designated flightschool (there's one in Orlando, FL) but the flight tests have to be done in the UK. Course, now I've typed all this the rules will have changed and it's all incorrect....but as of a few moments ago, I believe it to be true.
Hope this helps,
Dave