Oh man... one of my pet peeves is when people use the word "pwn" in real life. "Owned" or "ownage" also annoys me a bit, but not nearly as much as "pwn" and any of its derivatives do.
Generally speaking, I rarely use Internet slang in real life, unless I'm doing it on purpose to joke around.
Even when I write any type of digitized message (such as email, SMS, forum post, instant message, etc), I'll try to use proper English whenever possible. One exception is instant messaging, where I'll often forgo proper capitalization.
I'll sometimes use common abbreviations such as AFAIK, YMMV, BRB, LOL etc, but I try to avoid using short-hands such as u, y, r, tmr, 2, etc. With respect to the abbreviations, however, I will usually only use these in situations where I'm certain that the target audience will understand.