grammar policemen, more often than not, come across as petty.
I don't mind when people correct my grammar, personally, as I'm pretty pedantic about that sort of thing myself.
I go back, though, to something Smacks once said on a completely different topic. If someone's intent in correcting another's grammar is genuinely to promote proper grammar, then the person who does that is likely an idiot, as their repeated action inevitably does not achieve their goal with any frequency. If they're not an idiot, then they're lying about what their real reason for doing it is (in this case, they want to be a "dick").