Expect the web services to be offline between 10:00 PM and 12:30 AM Pacific Time. Apple doesn't mention if other services will be affected, so we assume you'll still be able to send and receive email via Mail for the Mac and iOS, as well as manage events with iCal. If you're restricted to using the web apps, get your work done early and enjoy a little forced down time later tonight.
For the latest MobileMe news and status updates, you can look here (the closest Apple has come to maintaining a blog). You'll find a history of recent status updates here.