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Apple Seeds Mac OS X 10.5.4, Final Release Expected in July
Apple has announced that MobileMe will be launched in early July. Apple's system requirements for MobileMe include Mac OS X 10.5.4 as well as iTunes 7.7, so we expect these two updates to also be released in early July.
Apple provides a Frequently Asked Questions document for existing .Mac customers who will be transitioning to MobileMe.
Update: The system requirements for MobileMe also list 10.4.11 as being compatible, but "some features require Mac OS X v10.5 and iLife 08."