I too am getting the blank App Manager problem on my N800.

It just started this morning.

apt-get still works fine. I removed all apps that I had installed yesterday, just in case, but App Mgr still doesn't respond. Blank screen only. I thought it might have something to do with my recent attempts to get SMB shares working, but I don't think so anymore.

If I try to close it, I eventually get the message, "Application Manager is not responding. Do you want to close it?"