Around the World

When traveling to Germany, will I be able to stick a German T-Mobile SIM into a US T-Mobile phone (it's got the 900 MHz frequency and has worked on T-Mobile's German network before, using the US T-Mobile SIM; but then of course I was paying the 99 cents/minute roaming charges) and use it as a local, German prepaid phone? I've tried sticking a German post-paid SIM in it on a previous trip, and that worked (this was my sister's, who lives there); I'm wondering, though, if maybe the German pre-paid SIM will register as a different provider. The phone is no longer under contract, so they won't unlock it for me.

I'd like to avoid buying another phone just for the occasional travel (even though they're pretty cheap at only 10 Euros or so - it'...
(continues)