What MAY have worked for me (currently assessing) was the following -
1. Delete all songs from Itunes library listing, keeping files in the music folder (ie, do not trash the files)
2. shut off match, quit itunes
3. reset my library according to the instructions here: http://support.apple.com/kb/HT1451
4. After completing step 3, readding my Music folder to itunes and letting it process all the music
5. Turning on itunes Match again.
Took some time, but seemed to work when nothing else did (safe mode booting, reseting the library WITHOUT removing and replacing the files in the library listing, and many other options suggested by Apple). I'm currently working my way through my library to see if it worked everywhere, but seems to be ok for now. Hope that helps someone else....