One reason I was stoked about iTunes Match was the idea I could have my Air anywhere and stream my music to it. I'd have my entire iTunes library with me everywhere, even without carting around my external media drive. It doesn't work. Why? Because if you are using a laptop on which iTunes thinks you SHOULD have physical tracks, it won't stream. It gives you the exclamation points and the "this file cannot be found, would you like to locate it?" error.
This is a giant, huge fail. I'm guessing I could get around it by creating a second iTunes library, logging into it with iTunes Match, and streaming that way, but I don't feel like it. It should "just work" the other way, I mean, the whole point of iTunes Match is you have all your stuff no matter where you are. Am grievously dismayed.