This is crazy - it's Saturday at 3:45 p.m. Arizona time (I'm in the Phoenix area). I started noticing this problem yesterday. Finally got fed up with it today so tried restarting, then checking Google to see if others are having this issue - and then I found this thread and others like it.
iTunes Match is great when it works - when it doesn't like right now, I start thinking about actually plugging my iPhone into my MacBook to (gasp) sync music with iTunes like I used to!
All kidding aside (although I really was about to manually sync my iPhone with iTunes to listen to some new tracks before I thought better of it), I had tried logging out of iTunes and logging back in - I see that solution had worked for some users but not for me and for others. Seeing the large number of people who have this same issue gives me confidence this is an Apple issue and not an issue with just my system or account. I've come to rely on iCloud more and more to sync Pages documents between my Mac and iPad, so this problem isn't exactly giving me a warm fuzzy feeling right now.
iTunes Match is a terriffic system, when it works - right now, it's just not working for me, and apparently, lots of others as well.
Come on, Apple - you're better than this!