I care, a lot, and especially on commercial websites. Poorly written ungrammatical copy suggests sloppiness and a lack of attention to detail and if someone treats their website in a sloppy manner I fully expect that they will approach other aspects of their business in a similarly careless way.
On forums it's much less of an issue, except when it descends into the total awfulness of text-speak, as people are often just shooting off their thoughts as they have them and mistakes are inevitable. It's more conversational.
"meetinged"! Now that's just awful, there is NO excuse for something like that no matter what the means of communication!