Just had the following experience when I was at the airport leaving the US:
I had got my T-mobile plan with my G1. When I was at a T-mobile hotspot, the network recognized that this was a T-mobile phone and allowed me to access the wifi network without asking me to login.
Bought an unlocked Nexus One, and put my sim chip in it. Now my Nexus One can see the T-mobile network, but it doesn't automatically get my wireless access. I get redirected to the T-mobile Hotspot home page and asked to log in.
Spent over 2 hours with the T-mobile support guys, but they couldn't help at all.
First time I have been disappointed with T-mobile's customer service. The supervisor didn't even agree to give me an account I could use before my plane left.....
Guess the good news is that it kept me busy for the 3 hours I was waiting at the airport..... :-)