At least, for the moment. Hackers are hard at work, and have been hard at work for a month now.
They've made good headway, we now have the ability to do 3rd party apps, and they can also do some simcard hacking. However, the ability to make and receive calls with a non-AT&T simcard is not possible at the moment. Another month or two, and probably, yes.
I have the Verizon 4 whatever iPhone which I do understand won't work in Europe. It works very well here, since I can get a signal (as opposed to ATT).
The question was supposed to be - can I get a SIM card/provider in Europe for my old 3GS ATT phone, which I'm no longer using but still have. So can I walk off the plane in Amsterdam or Oslo, find a SIM-card provider, and have them set me up with service with my old 3GS phone.