Just downloaded the iTunes update (had to do that manually from the iTunes web page; Software Update didn't see it) and it's "gathering information" about my library now -- fun to see how this all works out =)

edit:

After about 25 minutes of crunching it ended up matching a bit over half my library of 5,677 songs, having to upload 2,507 ... as I'm pretty sure my library is on the small side, I wonder if this scales reasonably to a larger, more "normal" one =)

/edit

edit 2:

Now I think about it, since it uploaded my copy of, say, William Shatner's version of "Lucy in the Sky with Diamonds," does that mean someone having the same version in their library won't have to upload theirs? Or will there be zillions of separate versions of that seminal classic?

IOW, if song X is uploaded once, does iTunes Match use that to serve everyone else who owns it?

/edit2