Vincent Everts managed to make a phone call (link in Dutch) on the network of T-Mobile in Holland using a Dutch T-Mobile SIM card. With that he’s the first person to get the iPhone to work on a network and with a SIM card other than AT&T’s.
The trick is to use a v1 SIM card. This type of SIM card is still rarely supported in Europe where most providers use v2 and v3 cards.
Here’s a video showing it working (notice the T-Mobile logo at the top left of the screen):
Instructions (in Dutch) can be found at iphoneclub.nl.