A search of Microsoft's own souce code will show significant amounts of BSD-licensed code (FTP.EXE is the classic example). This is entirely legal, but it's also possible that proprietary software contains illegally appropriated free or prprietary software.
In order to defend against SCO's claims, various people developed and refined the "shredding" method of comparing code for similarity. I don't know if this can also be run against compiled code, but it would be interesting to start running proprietary code against OSS products with similar functionality.

Even more intersting would be to find copies of actual MS source code. I wonder if anyone who has such would be interested in publishing the shred checksums.