This one came from Something Awful. In the FYAD section of the SA forums, someone posted a Warcraft III screenshot with the catchphrase in it. According to UrbanDictionary.com, this is what it said:

<omgwtf> dude, where are you?
<1337h4x> i'm in ur base, killing ur d00ds

I don't know where to find the original screenshot (searching the SA forums doesn't work very well since it was in a screenshot) so if anyone has the original and wants somewhere to upload it for us, let me know.

A recent hilarious exposure of the phrase came from this Photoshop Phriday (fourth one down): http://www.somethingawful.com/articles.php?a=1606&p=2

The SA Forums had an argument about this one here: http://forums.somethingawful.com/showthread.php?s=&threadid=934139

I think you'll need an account to read it, so the summary version of the argument is that it was from Starcraft, not Warcraft III. I honestly don't know since I got the Warcraft III information from a post on the Shack and I did have a feeling that I saw it it in a Starcraft screenshot. No one's been able to produce the original screenshot yet though :\

Just to settle the matter, it was from Starcraft, as it happened before Warcraft3 was even published.