First, the test realm forum is usually left up for a while after the patch so folks have some place to go and reference everything that was learned during the beta. I know that I've used the forum for up to a month to go and get necessary information.
Secondly, this comes less than a day after the World of Warcraft beta site experienced a change. When getting public releases of software ready, it's often a process of "First, get the site going – but don't link it to the public. Second, take down the public systems and get them ready." This is quite possibly what we're seeing here.
Finally, this forum always goes down before something beta-ish is released. This could mean they're just clearing it to implement a 2.4.2 release or something, but I doubt that given they've usually only cleared it for major point release. Ie: 2.2, 2.3, 2.4.
An interesting tid bit of WotLK news for you all this morning. I'm going to go out on a limb here and say that within a month we'll see some major announcements concerning the next expansion.
Gallery: Wrath of the Lich King