I go through phases with that one. It irritates me to no end.
I have a special pet peeve for the whole their, they're, there and your/you're. especially the latter.
Wow, you guys keep bringing up ones that totally get to me too. Again another one I am sure most people are irritated by.
I know a lot of people get worked up over the whole who/m, however I still have no idea which to use when.