I don't even think about it. I work in IT so my life is chock full of acronyms and I guess I'm just used to them. They don't usually trip me up.
I do hate it when people type in "text speak" in regular email though, especially at work. If I get an email at work where someone is typing things like, "I have a file 4 U" I will respond and ask what a "File 4-U" is and why they're telling me about it. I'm at a level now where I can be a bit of an old crank about stuff like that! I don't mind things like "LOL" but 4 instead of "for" and U instead of "you" is too much!