When I unlock my iphone can I use a prepaid SIM card when I travel?

I live in Germany and a friend sold me her iphone 3gs when her contract was up. She said the phone would be unlocked after a certain date but it hasn't yet. If i do this myself will I be able to use any SIM card? I have 1and1 for phone service and when I travel to the states I get a prepaid SIM from tmobile. Will these work on the iphone too?

Yes you would. That's the benefit of an unlocked phone. Any SIM card will work.

Yes, Once you have unlocked your iPhone you can use any international GSM SIM to it. You can unlock your iPhone through remote unlocking method. It is the safest and easiest method to do so. It is also an affordable method too. You can get the remote unlocking for an iPhone from Wickedunlock.com All the best mate.