I realize I post this at great personal peril, but Mobile Me appears to be working. Apple has updated the Mobile Me support page with information specific to Mobile Me, and has removed the notice about the update taking longer than usual.
Even so, Internet Explorer 7 users appear to be left in the lurch: Apple displays a notice for those users saying IE7 is "not fully supported." Apple mentions on its website that Mobile Me can sync IE7 bookmarks, and features the IE7 logo fairly prominently.
Your mileage, as has been the case for the last 48 hours, may vary. It will be interesting to find out in the coming days what precisely happened, and what Apple will do to prevent it from happening in the future.
Thanks to everyone who sent this in!