could be a hardware issue. i had the same problem with my Sprint TP2. 3 different computers wouldn't recognize the phone when connected. Tried all the tips/tricks and nothing worked. I was able to sync via BT.
Sprint replaced it. There are a few threads and a poll regarding this issue.
Here is the thread i started about my problem
http://forum.ppcgeeks.com/showthread.php?t=105233
Good luck!!