DB - here's the procedure
1) Read
http://www.faa.gov/licenses_certific..._verification/ submitting the form which it links to.
2) At the same time send
http://www.caa.co.uk/docs/33/FORSRG1160.PDF to the CAA (current charge is £39, use
http://www.caa.co.uk/docs/33/FORSRG1187.pdf to make payment)
I would recomend sending all by fax, not snail mail.
To find the most convenient FSDO do a search on
http://www.faa.gov/about/office_org/field_offices/fsdo/
When you have your letter back from the FAA saying you can have a certificate present yourself at the nominated FSDO (Give them a ring just before so they're expecting you) with your UK licence and medical certificate and they'll finish off the paperwork, giving you a temporary US certificate (your permanent one wil arrive in the post).
It really is that easy.