Originally Posted by canred

The UK automated system (IRIS) is not limited to UK passport holders. You are allowed to apply for IRIS if you meet one of the conditions shown in the attached link:
http://www.ukba.homeoffice.gov.uk/ma...iregisteriris/
The most likely one for US passport holders is the last bullet item
"are a short-term visitor to the United Kingdom and can show you have been given permission to enter the country as a visitor at least twice in the past six months, or four times in the past 12 months at the time of registration"
which is aimed a frequent flyers.
The application process is very straight-forward and you do it at the airport on your way out of the country (after you have cleared security). This thread has more info:
http://www.flyertalk.com/forum/briti...ring-iris.html