I'd been debating, for a while, getting iTunes Match to have cloud access to my music (and better-encoded copies of a few songs). Well, finally broke down and did it a day and change ago.
So it looks like the first thing it does, when you sign up, is to identify every song in your library (in my case, a bit more than 22k tracks). I'm guessing it's matching the music itself, rather than the metadata (and there are good reasons for doing that, of course, though some pitfalls as well).
Anyway, I wasn't aware of that, before signing up. If the progress bar is to be believed, in the 30+ hours since signing up (and with a 35Mbps (theoretically) pipe), the process is less than 40% done. This is especially annoying as I now have some good uses for it, and want to try it out.
We'll see how it works out once it finishes, I guess; I don't see any sign of iTunes Match in my phone yet. Hopefully it will appear in some useful format.