Yes you can play PAL games on a US PS3. This applies to PS3 games only though. All PS3 games are currently region free whether they be US, Japanese or UK. More then likely they will remain region free in the future.
By the way if you had done a search then you would have found several threads on the subject. No real reason to start another one.
Yes it does work the other way too. Although all the games have region coding on the back of the cases it doesn't mean anything just like with PSP gamers. All PS3 games are region free regardless of where they are from.