What should I do to be able to log in to MobileMe without Unknown Validation Error?
I was having trouble logging into my Apple account so I could send a card from the Cards app earlier today. It eventually worked, but only after a ton of tries. Since it all uses the same validation service, there is a possibility that it is currently undergoing maintenance. Try again in a few hours and see if it is still not working.

http://www.apple.com/support/systemstatus/



Apple has a nice service status page you can check, but my experience is that sometimes the Authentication servers are overloaded or not working even when all the lights on the dashboard are green.