It all started with emoticons. Now, every marketer that has a high-school or college audience thinks all they have to do to connect with those crazy kids is put their message in IM/texting shorthand. If you’re going to do it, it has to be done very well (let your intern take a shot at it, at least they’re up to speed), and it has to be relevant (good for cell phone marketers, bad for cereal). If you don’t do both of these things, your attempt will be just about as cool your dad walking in with baggy pants, covered in bling and saying to your friends, “Yo kids, what is up? It’s okay, I’m hip, I’m down, what’s the 411?” When in doubt, don’t do it. It’s already overstayed its welcome.
But, if you want to reach the gamer crowd, especially the online gamer crowd, 1337 sp34k may still have some shelf life left. According to Wired, it's even gaining validity.