[EVOTION]

There are indeed some games that even if you are in Europe will ONLY allow you to play Americans. An example is COD3.

Some games are Global like COD4 which means it doesnt matter which regions' disk you have in your PS3 as everyone ends up on the same server.

HOWEVER.... you may find that if you have a US copy of these games and are playing in Europe that lag may be worse as you 'may' be routed differently.

Conclusion

Only buy games from your region!!
Its not worth the extra savings buying an import.
Also, you will almost certainly find your games can NOT be traded in or sold in your region if they are from another region!