Check out the thread I did on this very topic:
http://www.camaro6.com/forums/showthread.php?t=471821
I painted the inside of my exhaust tips well over a year ago and they still look as good as the day I did them. I'm quite happy with the look and definitely happy about the labor I now avoid keeping them clean. Just a quick wipe when washing the car and I'm done.
Note: I also painted the mufflers black as well to make the trash can look disappear.