When Apple announced iTunes Match, they originally hoped to have it available by the end of October. The service scans the songs in your music library, including music not purchased on iTunes, and matches them to the more than 20 million songs available on the iTunes Store(R), offering them in high-quality, DRM-free 256 kbps AAC encoding. Any unmatched songs are uploaded to iCloud so you can play songs, albums or playlists from your music library on your devices.
Well, they didn't quite meet their own deadline, but Apple has at long last released iTunes 10.5.1 and launched the iTunes Match service for $24.99 per year. The download is available now at http://www.apple.com/itunes and should be available shortly via Apple Software Update.