I know of nothing as simple and streamlined without using a mixture of other free services, such as Google Calendars, which is messy and never works how you want it with dotMac.
Obviously now the question is can this be done with MobileMe. With it's "Exchange-like" features, surely it is a lot more doable than before. Maybe if Apple released some info on the server details we could use ActivSync on wm6 to connect to it as if it were an Exchange (using the Outlook Web Access address works beautifully).
Anyone else know of anything out there now MobileMe is here?