Preview at WWDC likely in June, followed by September launch.
Apple Launches iTunes Match with Release of iTunes 10.5.1 [Updated]
Apple had apparently initially intended iTunes Match to launch by mid-October as part of iTunes 10.5 as seen in a number of developer betas, but the company removed the feature from iTunes 10.5 for the public release, giving it a few more weeks to complete work on the service. Even so, Apple missed its stated launch target of "end of October" and did not provide an updated timeframe for the iTunes Match debut once the target was missed, although the company continued testing with a new developer beta of iTunes 10.5.1 just this past weekend.
iTunes Match is a U.S.-only service for the time being, although the company is reportedly working hard to secure licensing to details to expand availability to other countries. iTunes Store users in a number of countries have been greeted with a "coming soon" tag on iTunes Match promo listings in their country-specific stores, suggesting that Apple is optimistic regarding expansion of the service in the relatively near future.
Update: Demand of iTunes Match subscriptions has overwhelmed Apple's systems, and the company has halted new signups for the time being.
New iTunes Match subscriptions are temporarily unavailable.Update 2: Despite Apple's notice about overwhelming demand still showing on the iTunes Match subscription page, subscriptions do now appear to be going through.
Due to overwhelming demand, iTunes Match is not accepting new subscriptions at this time. Please check back in an hour.
Update 3: iTunes 10.5.1 is now available in Software Update.