Yes, just insert SIM. Turning off 3G is simple. Settings/Network/General.

But that's not what you want from what you said, you really want to turn off data roaming, location services, and make sure you don't use push settings for email or any apps if you want to control your data usage. You need to do a bit of research for that or you could end up using a lot of data.

InfoMofo: UK had factory unlocked (sim free as they say there) iphones from day 1. USA has them now too.

-David