Ars Technica has a great overview of MobileMe and the switch from .Mac to MobileMe which is currently underway (as I type this).
If you’ve been following the updates about what MobileMe will do and what will fall by the wayside as .Mac is retired then much of the overview will be familiar to you. There are a couple interesting quirks about the transition though. For one thing there is an IM implication for people who shift to me.com from mac.com (it may be a reason for you to stick with mac.com). It is also a little strange that MobileMe doesn’t synchronise bookmarks.
The update has apparently been pushed back by 2 hours and should finish around 11:00 local time (if iCal is getting the time zone thing right, that is).