You most definitely can do this (from any carrier). Check out the HoFo TMO prepaid forum for more info. http://www.howardforums.com/forumdisplay.php?f=343 The guys over there know every nuance of using prepaid.
To the poster who said he wasn't allowed to port from AT&T post paid to TMO prepaid, you got bad info from someone. Others have done it without a problem.
Wow! The mods are working fast. I had barely posted the above and the post was moved to the prepaid forum.
Personal Phone: iPhone 4 on AT&T
Work Phone: Blackberry Tour (9630) on Verizon