Apple has released iTunes 10.5.1, which comes with iTunes Match. The iTunes Match service, which costs $24.99 per year and is currently available only to U.S. customers with fewer than 25,000 tracks in iTunes.
How it works from Apple:
iTunes determines which songs in your collection are available in the iTunes Store. Any music with a match is automatically added to iCloud for you to listen to anytime, on any device. Since there are more than 20 million songs in the iTunes Store, chances are, your music is already in iCloud. And for the few songs that aren’t, iTunes has to upload only what it can’t match. Which is much faster than starting from scratch. Once your music is in iCloud, you can stream and store it to any of your devices. Even better, all the music iTunes matches plays back from iCloud at 256-Kbps AAC DRM-free quality — even if your original copy was of lower quality.