Take the card out and put it back in. Or you could put a small piece of tape on the back of the sim card so that it will "push up against" the phone nice and tight when you slide it back in. If that doesn't work...RESTORE.
Well you haven't restored to fresh new 2.1 since you posted, right? That should fix it all. Just do a complete restore and then re-jailbrake...I know it's a lot of trouble but this will fix the problem.