Apple will stop selling the .Mac service, existing .Mac users will have their @mac.com email addresses forwarded to @me.com.
"MobileMe is slated to include a host of new features, in addition, there will be new web interfaces for all aspects of MobileMe -- calendars will look just like iCal, Contacts will look just like they do in Address Book, etc. This is similar to the way .Mac Webmail works today, though we've heard that the new interfaces will be much snazzier (yes, that's a technical term). Apple should also be updating the .Mac dependent applications (iChat, iWeb, iPhoto etc.) to work with the new service. The new service will be backwards compatible with .Mac protocols for the time being."
MobileMe is expected to be available in late June/ early July.
MobileMe is already built in to the latest beta of the iPhone OS.
MobileMe will feature over-the-air syncing of contacts, calendars, and photos.
A version of MobileMe for Windows is planned.