Preview at WWDC likely in June, followed by September launch.
MobileMe Launching on July 9th Between 6pm-12am PT
According to their service status update when you log into .Mac, they detail that MobileMe will launch on the evening of July 9th:
7/9/2008, 6pm-12am PT
As part of the MobileMe launch, www.mac.com will be taken offline at 6pm PT on Wednesday, July 9th.
Members will be unable to access www.mac.com or any .Mac services during this time with the exception of .Mac Mail accessed via a desktop application, iPhone, or iPod touch.
MobileMe will be available as soon as possible during this maintenance window.
Existing .Mac subscribers should already be able to send and receive emails at their new @me.com addresses. MobileMe retails for $99/year, but iPhone 3G (or Mac) buyers will be able to pick up a year subscription for $69 on July 11th.