This comes after .Mac has been rumored to undergo re-branding around the time of WWDC (which is next Monday).
Update (8:38p EST): Apple just updated the system status to report that some may not be able to access the .Mac welcome page. Stay tuned to TUAW for updates on the .Mac outage.
Update (9:37p EST): Apple is telling users that they can access the .Mac mail by using the webmail system; however, most users cannot access the entire .Mac website. We're currently experiencing a "504 Gateway Timeout" error when trying to load both the homepage and webmail. Looks as though the same services are down.
Update (10:40p EST): Some commenters are noting that they are now able to retrieve their .Mac mail. Apple has yet to update the .Mac system status.
Thanks to everyone who sent this in!