No tricks are necessary. It should be set up as follows:
1. Encryption disabled
2. Infrastructure ticked (as opposed to ad-hoc)
3. Set the SSID to "tmobile" (without the quotes) or let your card autodetect it
It should connect after at most a minute or so; then a menu should come up asking you to type in your details (as far as I remember - it is still free here in the UK so I have only done it once).
If that doesn't work, try "t-mobile" (we used to write it like that).
It should be very stable and fast too! At least in the UK where very few people use Wi-fi.
Jorgen