I tend to run into this issue every time I do a production deployment on this one particular MOSS farm.  What happens is when I deploy a solution, stsadm of course says everything worked great but I try to look for my new code and its not there.  Eventually, I remember to look at the solutions page in central admin where I will always find an error.  It turns out on a few of the frontend servers, the solution deployment failed because it can't update one of the DLLs in the bin directory.  It's the typical, this file can not be copied because it is in use error message.  After going through the list of processes, I finally found the one that was the culprit SP2K3DocBackup.exe.  After a little research, I discovered this belonged to CommVault Galaxy (a data backup application).  The solution was simple, once I killed this process on each server, I redeployed my solution file and it installed correctly.  I am not sure why this process locks your DLLs.  You would think it would use Volume Shadow Copy or something so it wouldn't have to.  Hopefully this will help if you run into this situation.