As far as I'm aware it's the first time it's ever happened here.
I was online and MSNed a couple of the Senior Admins as soon as I got the warning.
It was dealt with so that quickly that no one else outside the staff even knew about it but I know that the other staff investigating it, without Norton AV installed, queried it before it was confirmed that we had in fact been hacked.
The hackers inserted obfuscated java code that was intended to redirect users to another website but it didn't work properly and they didn't have time to exploit it.
As a side effect it did mess up several sections of code however which took some time and effort to fix, such as the style chooser for example, but other than that there were no ill effects that we're aware of.
As far as your AV protection goes, there's nothing special to scan for as we caught this exceptionally quickly, the inserted code was disabled almost immediately, and the system was patched not long after to prevent this particular exploit.