A couple of times over the past year, I have somehow broken Metasploit on a Backtrack 4 install. This can sometimes happen if you lose an internet connection whilst running msfupdate.
When this happens, Metasploit will fail to start, with some nasty error messages, basically saying there are file mismatches or missing files. Metasploit will no longer update using msfupdate, and this can be a pain if you have a HDD or USB install of Backtrack.
Here is how to fix this issue, by reinstalling MSF from the Metasploit repository (pretty simple, delete the current Metasploit install, and reinstall)
rm -rf msf3
svn co https://www.metasploit.com/svn/framework3/trunk msf3
(Choose "p" if it asks a question about trusting the repository)
After it has finished updating, you will have a working Metasploit Framework again. Hurrah!
(Note: /pentest/exploits/framework3/ is only a link. The real location for the Metasploit install is in /opt/metasploit3/msf3)