What sort of encryption are the connections using? I know when i got my n900 i could not connect to wifi that used WPA encryption but i could connect to wifi using WPA2 encryption. Might try changing encryption type (even try removing all encryption just to make sure ur wifi antenna is working).

As for your tmobile problem, i had to give tmobile customer service a call when i bought my phone. I had the correct setting in the phone but they had messed up on my account and thats what prevented me from connecting to their data network.