Isn't that what the 2555-EZ is for? You only get $80k tax free though.
You have to deduct the days you were physically present in the US, even holidays (I really hope this is wrong, as it seems unfair, but I have yet to hit the $80k limit, particularly with the 5000 deduction from line 40, but even so, it's just wrong ).
There is a place to get the exchange rate, but can't remember where. I always have to look it up.
So, say you earned £30k and were in the states for 14 days. (say, £30k*1.6=$48k)
line 14. 365-14 = 351.
line 15. 351/365=.962
line 16. 80,000*.962= $76960 (your excluded amount)
line 17. 48,000
line 18. (smaller of 16 and 17) = 48,000
Then on your 1040:
line 7. 48,000
line 21. (48,000) then put 2555-EZ on the dotted line.
line 22. 48,000 + (48,000) = 0
Job done. I'm guessing the assumption is that, as you submit your P60 (w2) with it, you HAVE paid the tax in the UK.
Note: I was looking at a copy of my 2005 forms when I filled this out, the numbers may be different now!
2nd note: Yep, I've just looked at my 2007 forms and it's gone up to 85,700.