I had a problem this morning being unable to delete a file (on a NAS drive) because Finder said it was in use... rm gave the same error, as did trying to eject the drive to force it out of use (when in doubt, get a bigger hammer ).
It shouldn't have been in use... I sorted it out eventually by rebooting the iMac, but it's a bit brutal. How can you find out which process Finder thinks a file is being used by? (I assume looking the PID up in Activity Monitor will give the application)
And if that gives no useful result (ie OS X has got it wrong) is there any reasonably safe way to delete a file regardless of its "in use" status?