I have traced references to this to 2004, but it may be older than that. According to the wiki spoof site Encylopedia Dramatica it originated as “Im in ur base killin ur d00ds”. Sources around the web believe that the phrase appeared on a StarCraft screenshot in the Something Awful forums,
but if it did, it can no longer be found there or anywhere else on the web.
(Note it’s “all ur doodz” rather than just “your doodz”)
Ask the average internet user and they’ll probably be able to produce a variation on this snowclone and give a sense of where it comes from: gaming chatter, leetspeak, etc. This is, I think, what makes this phrase a snowclone–it is meant to be funny, but it isn’t funny if you don’t know anything about the world in which it originated. Since the content words are all variables, the meaning of this phrase isn’t necessarily limited to “LOL you got pwned and don’t even know it yet”. That is, if you write, “Im in ur meeting planning ur barcamp”, you’re not saying that you’ve taken over the meeting. You’re making a literal statement about where you are or are going to be and how geeky you are.
Another change to this phrase’s “original” usage is its entrance into kitty pidgin. “Im in ur X Ying ur Z” is very popular on the macros that have become known as LOLcats. Variants:
Spelling in this snowclone can vary: “ur” may be “yr” or even “your”, and “Im” may be “I’m” or “I am.” (Though I find the latter suspect as true kitty pidgin, unless it’s part of the separate meme of “correcting” LOLcatese. [“I am in your forum posting in a grammatically accurate manner.”])