yah, been doing this for a while now.
if you switch over to MCI, within a couple of weeks, you will get a letter in the mail (i always do at least) from AT&T with a $40 check if you switch over to AT&T. then if you switch over, MCI will call back a week later (like they called me again two nights ago) offering $50 in long distance to switch to MCI. i tell them "but AT&T offered me a check, i would not like free long distance, i want a check" and they say "no problem, we'll give you a $50 check and another $5 check to cover the switching fee"
AT&T doesn't cover the switch fee, so you have to absorb that $5 yourself. but once you (i don't know how i got so lucky to, only been here 6 months and not done anything pro-active to cause this to happen) get on their lists, you can keep bouncing back and forth and making money on long distance to help pay for those damned PacBell or TimeWarner bills
enjoy,
Shang