According to Apple's new MobileMe site, most of MobileMe, including email, Find My iPhone, and everything from iSync, will remain intact. iDisk, iWeb and Gallery, are gone, but they'll be replaced by iCloud features with different names.
Some serious stuff is going away forever, though. Like everything you've uploaded to Gallery and iDisk. You'll still have access through June 30, 2012, but anything you haven't downloaded and saved by then will be lost forever. And if you use MobileMe's syncing services to sync Mac Dashboard widgets, keychains, Dock items or System Preferences, you're SOL after June 30, or whenever you switch to iCloud.
Most frustratingly of all, despite having the option of porting your MobileMe email address to iCloud, you will not be able to merge it with an existing iTunes account. Apple says you can still use different accounts for purchases in iTunes and iCloud. Gaaaaaaaahhhhh. [Apple via 9to5 Mac]