I have 847 waiting right now. It says it's uploading one of them in iTunes, but the bar hasn't moved in a while. I'm guessing the servers are just being hammered and it's going to take a while to upload the songs it couldn't match.
I've had one song "waiting" but it hasn't changed in any way since I started Matching songs. After I fixed errors and ineligible songs, I Matched everything all over again, but the same song is still "waiting." What's going on!?