I'm having the same problem.
I have 2 iPod Touch (both 2nd Generation). I installed 3.0 on both, then jb both with redsn0w. Cydia is working perfectly on one, the other will not boot Cydia at all... it keeps acting like it's going to launch, then jumps back to the springboard. I've rebooted several times, and even reinstalled 3.0 and the jb. Still the same problem. 🙁
Anybody have any suggestions?