When I upgraded to Tiger, I did a clean install and used the migration assistant to copy over my applications and settings.  Unfortunately this copied over some Safari plugins (such as PithHelmet) which trashed Safari.  I looked up how to fix this, and was told to delete my SIMBL directory (I'm not at my Mac but I believe it was in Application Support or something like that).  I did that and Safari worked fine.  Unfortuately, now I can't empty that SIMBL directory from my trash because it is "currently in use."  Its even currently in use if Safari is not running.  How can I find out which process is using it?