The common factor seems to be that this is affecting people trying to login with email@example.com credentials.
Now iChat attempts to authenticate against login.oscar.aol.com (port 5190 by default, you can also use 443) and that machine appears to be up and running just fine. Indeed, people with AIM accounts seem to be unaffected, which implies that local authentication against AIM/AOL's own user database still works.
If, and this is supposition on my part, on handing login.oscar.aol.com a set of .Mac credentials then a connection is made behind the scenes from AIM/AOL to Apple's user database for .Mac accounts then the failure to login suggests to me that the problem is with the remote authentication service that Apple provide to AIM/AOL.
When I got this error yesterday, I spent some time investigating and discovered that the WebObjects gateway on www.mac.com was failing with a 504 Gateway Timeout error. Although this came back up about 20 minutes after I tested it for the first time, and allowed me to login to my .Mac account on www.mac.com, the iChat login problem has remained.
So what's going on? Apple aren't saying anything (at least nowhere that I can find) and the AppleCare Support phoneline's only suggestion was that I report the issue via the iChat iChat feedback page (which I've done).
If Apple want to be taken seriously (say, as seriously as Google) as a provider of online services - and .Mac is one of their flagship offerings alongside iTunes - then they'd better improve their system status reporting a bit.
Here's the thread on Apple's iChat forum.