MobileMe is slated to include a host of new features, which we alluded to early in May; 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 -- so developers will not have to rush out updates as soon as they hear the official word.
MobileMe is scheduled to be available sometime in late June/early July. We have also heard reports that the latest build of the iPhone beta firmware (2.0) has already undergone revisions to handle the new MobileMe service. Some of the features of the service on iPhone will include: over-the-air syncing of contacts, calendars, and photos. Interestingly, there is also a Windows version of MobileMe planned.
Though we're confident in the source of this information, this is still a rumor until Apple reveals all. We should hear more information about MobileMe at the World Wide Developer's Conference starting on Monday (June 9th).