Does Sophos Antivirus work correctly on Lion with (legacy) FileVault enabled?
You need to increase the number of on-demand scanner threads, to prevent a deadlock when you log in if you have a number of things all trying to start at once.

Run the following, from Terminal:

sudo defaults write /Library/Preferences/com.sophos.sav WorkerThreads -int 20


(20 works for me, with the number of applications that I have set to launch at login time)