To be really lame and get techinical on you guys, there is a different between 1337-speak and chatspeak.
Leet-speak originated from gaming. Alot of it is short-hand words that sometimes use numbers (but whether that's because it's quicker, or it looks cool, I don't know). Examples are 1337 (which means cool, for some reason), w00t (yay) and pwned. Pwned came from a misspelling of the word 'owned', used much in the same way as the term 'schooled', to mean dominance over somebody, specifically in a game. (ex. "I beat your character at the last second; I pwned you!")
Chatspeak is the short-hand used most often on instant messaging programs. lol, rofl, brb, g2g etc. It can be useful when in a hurry or if you only have a short message to convey, but I do agree that it is really annoying when people use shorthand constantly in the place of typing out actual words. For example when tweenagers and like people use weird, phonetic shorthand - "lyk" instead of "like" or "gr8" instead of "great". That's just getting kind of lazy.
So yeah. Both are annoying and don't make much sense, and aren't particularly useful, but there you are.
And, if you don't like this stuff, I can always just SYFFITF!
(Hee hee **giggles**
)