Don't forget, Freddie - most new phones that you buy (from T-Mobile and Cingular for that matter) will be "locked" to the carrier for X amount of days after you buy it. This means, that you can't just purchase another SIM card (from Vini for instance) and use it in the phone until you get past the lock out period. This is because the phones are subsidized by the carrier - they don't want you to purchase the phone and then run away to another carrier to use it on their service. It's possible to buy "unlocked" phones to get around this problem but they will cost more.

Masaki