I am also having this problem. Seems like it might be connected to an issue where I can't log into iCloud in the System Preferences panel. I either get an error that my password is incorrect, or it times out after 10 minutes of the spinning wheel of doom. Also running Mountain Lion on a new iMac.
Even after changing iCloud password, I still can't log-in on the iMac. Syncing my password on my devices allows my email to work, but on the desktop at home I can not log in no matter what.
iTunes match continues to try to upload a couple of tracks purchased outside of itunes, but cannot complete even the first upload before it re-starts.
I've seen a couple of posts on this, but no one seems to have an answer.
I have had terrible problems with the service. I also had about 4,000 songs unmatched. It took me two weeks to get everything uploaded, and I'm surprised it was that quick given all the problems I had. It tends to get hung up after a while and just stops uploading. I have found that if you sign out of itunes match, shutdown itunes, and preferably restart your computer, then turn itunes match on again, it often seems to help unstick whatever problem seems to be occurring. With that said, I have a few longer tracks (one track mixes that are 85mb to 100mb) that I just cannot get uploaded to itunes match. It is really obnoxious. I can't solve the issue. Either it is on apples end, or there is something wrong with my ISP or it's a networking issue beyond my technical abilities. It is really frustrating! Hang in there. Once things are in the cloud the service works for the most part. I would have given up on it, but my new MBP only has a solid state drive, so there is no way for me to physically store my music on my computer anymore.