OK, interesting little issue we came across today. Figured I'd share this in-case anyone else is going through the same unsettling couple of hours we just went through.
Our company has a Sonic firewall. The people over at Sonic release security updates (logical); in particular this one :
SonicWALL - MS IE mergeAttributes Function Invocation (http://software.sonicwall.com/applications/ips/index.asp?ev=sig&sigid=3099)

This is to patch a known vunerability in; surprise, surprise Internet Explorer :
(MSDN:: http://www.microsoft.com/technet/security/Bulletin/MS10-002.mspx - 21JAN2010).
Unbeknownst to me this occured at 16:00 yesterday (26th Jan 2010) automatically. After receiving a phone call this morning; pretty much to the affect of : "the website is down", spiraled a series of events that ended up with me looking for Javascript object/variable conflicts in 3rd party supplied vendor code.

Using the savior that is Firebug (thank you Joe Hewitt and a dedicated community) I was quickly able to isolate the problem to a Javascript runtime error, in this particular case; the Mootools Fx() class. Because of this 'error' Mootools failed to run/compile and thus began a progressive series of 'domino effect' events. Mootools didn't execute window.addEvent('domready',function(){}), our AJAX content didn't execute and the page simply failed to load ("the website is down") ... I began having nightmares. Was Mootools now flawed because of some random browser update?

After quickly testing and noting that every single website I'd created using the Mootools library was also failing on the corporate LAN I began to feel a little light-headed. A hasty call to my wife, God Bless my external tester, quickly calmed my nerves after confirming everything worked perfectly outside of the LAN. OK, so it's internal only.

More frustratingly all the code worked flawlessly in our production environment. At this point I'm thinking firewall. "Any recent changes?" Turns out there was one... yesterday at 16:00. OK.

I'm not sure entirely what my firewall partner-in-crime did but it worked, altering the level of action taken on detection of this alert seemed to re-enable the site. From what I can understand of this threat is that if the firewall detects attempts to exploit this Javascript vulnerability it prevents the code from executing, in this case 'causing Mootools to fail and the website to simply 'not show up'.

I suspect it's because of our firewall configuration where we actually have to go outside of the firewall to come back in again. Any AJAX requests (or in this instance Mootools' new Asset.image() class/method). Content/Javascript is theoritically going outside of the firewall only to come back in again, thus 'causing the Firewall to detect this as a threat. A point possibly confirmed by the fact that this issue was only noticeable on websites where AJAX methods had been adopted to render data.

Anywhere; we're back in business, but I figured I'd share in case anyone else is currently looking for answers to this particular problem.

I've also posted this to the Mootools google.group for further incite from others. Follow the thread here