Eh - what the hell is this crap? A bug in MOSS 2007 stops you deleting .master copy files.
Apparently its because when you reset a master page to a site definition, MOSS somehow disconnects the copy file from the sharepoint server - consequently you cannot delete it and get the very annoying "This item cannot be deleted because it is still referenced by other pages"
Well, the KB article about is plain incorrect, and only suggests that you can hide the file from future sites - and let your crap grow infinately every time you reset to a definition.
http://support.microsoft.com/kb/926812
There is a very easy workaround it seems - using sharepoint designer, create a new folder and move the culprit files into a new folder. Delete the folder and the files magically dissappear - no hiding, no frustration.
I've heard this also works for other MOSS file types locked in the same limbo hell.