As part of Apple's iCloud service announced today at WWDC the company also discussed iTunes in the cloud. iTunes in the cloud lets you download your previously purchased iTunes music to all your iOS devices at no additional cost, and new music purchases can be downloaded automatically to all your devices.
Apple also covered iTunes Match a feature which allows music not purchased from iTunes to be replaced with a 256 kbps AAC DRM-free version if we can match it to the over 18 million songs in the iTunes Store, it makes the matched music available in minutes (instead of weeks to upload your entire music library), and uploads only the small percentage of unmatched music.
iTunes Match will be available this fall for a $24.99 annual fee and will be a US only service.