Ok, I have an AT&T Tilt as well and I use T-Mobile. What you need to do is Unlock your phone completely, meaning sim/cid unlock, you know what, go here and all the instructions will be in the sticky's
http://forum.xda-developers.com/forumdisplay.php?f=378. Then, if you want to flash to a different rom to get rid of all that at&t bloatware crap, again read the stickies in the link I provided. Flashing roms is the fun part.
