Just got sidekick and using it a t&t plan -

find someone with tobile who has been a customer for more than 90 days.
then have them to call in
tmo is legally obligated to unlock it
but ppl reciently says it cannot be done
im having the same situation
but i been doing tons of reasearch
it is possible
but do not use webisghts who says if u pay them 5 or 6$ they will send u a code it doesnt work
best bet is call tmo and ask
if not there is this thing on line u can buy for 9$
that u put on the back of u sim and it ovverrides and confuses the sidekick and will let u use ur att sim