Actually, the English use of "woot" predates both gaming and hacking... by about 600 years.
As an English verb (it was called English back then, but we call it Middle English now), "woot" was a variant writing of "wot", the first or third person conjugation of "wit", meaning "know". (See preterite-present for more information on these sort of verbs.)
"Woot" was chiefly used by Chaucer. Consider (SPOILERS!) King Alla's exchange with the Senator when unknowingly meeting his own son in The Canterbury Tales, The Man of Law's Tale: (Chaucer, 1400)
"Whos is that faire child, that stondeth yonder?"
"I noot," quod he, "by God and by Seint John!
A mooder he hath, but fader hath he noon,
That I of woot." ...
...
"But God woot"
"Whose is that fair child, standing yonder?"
"I know not," said he, "By God and by St. John!
A mother he has, but he has no father
that I know of." ...
...
"But God knows"
As you will note from the above passage, the negative of "woot" was "noot". This was the common spoken contraction of "ne woot".
I modestly propose that we use "noot" in its original intention, as a negation of "woot". Thus, when fragged cheaply by a friend, proclaim "noot!" in the face of his exclamation of "woot!"
As for the etymology of the modern woot, w00t, wewt, etc. used as an exclamation of joy, I had always just assumed it was a crossing of the words whoop and hoot -- as in to "whoop for joy", or "hoot for joy". But this is just personal conjecture. (UPDATE: Hey, I just noticed that our good friend Webster has whoot as an obsolete form of hoot -- I win!!)
(FUTHER UPDATE: I think sniepre is onto something -- anyone remember the Tag Team song, Whoomp, There it Is?
(YET ANOTHER UPDATE: w00t has just been selected as Webster's 2007 word of the year. How much do you want to bet that the selection is based on an online survey? http://www.merriam-webster.com/info/07words.htm )