Depends on the studio, some USA titles are A,B,C and some aren't. Best to check each title as you may be lucky, though Rati is region A!!! I wanted to get it when I was on holiday in Orlando.
ive had this problem with an import of Superbad on my ps3. Ive got three US games that work fine and also two other blu rays. When i put Superbad in my ps3 tho the screen says "region code does not match disc". Wonder if its something on a firmware update?
Check out this site http://bluray.liesinc.net/ It will tell you which imported BDMV disks will work on your UK PS3. BTW Superbad will be released on Bluray in the UK on the 21-01-2008.