Apple has finally posted iTunes 10.5.1, which is the first version to enable iTunes Match with sync via iCloud.
iTunes Match is an optional $24.99/year service which takes songs you’ve imported from CDs or purchased from Apple and stores them online with iCloud. Once enabled, you can play your songs on any iPhone, iPad, iPod touch, Mac, or PC without syncing.
iTunes will determine which songs in your library are available in the iTunes Store. Then, any matching songs are automatically added to iCloud. Any songs that don't match Apple's 20 million song catalog, will simply be uploaded to iCloud.
Perhaps best of all, the music matched in iTunes is played back from iCloud at 256-Kbps AAC DRM-free quality. This is true even if your original was encoded at a lower quality setting.