The web interface seems to be working faster than the Apple Mail app, but even the web is slow. No communication from Apple support about what the problem is, but something clearly is wrong for some customers. Others may be doing fine.
I'm not seeing any reports from users on the Apple support board yet, but I expect they'll be flowing soon. Several MobileMe customers are seeing repeated timeouts from Apple servers.
Hopefully this will be just a short lived glitch, and not an epic failure.
Update: Service seems to have been restored for as of 3:15 EST, but still no word on the MobileMe support site.