Phil Ringnalda linked an interesting post about who owns blog comments. While I agree that the commenter owns the copyright on their words, I think that by posting them in a public place commenters lose their control over that particular copy.
I think of this as the “bathroom wall” principle. If you write a poem on a bathroom wall, the owner of the restaurant isn’t obliged to erase it at your whim. Nor should he have to let you back into the bathroom to edit it yourself. He can stick a neon Guinness sign in the bathroom next to your poem. If he happens to hoist his double-wide restaurant onto the back of a trailer and move it to another city (or country), then the bathroom goes with him and he’s still not obliged to do anything about your poem.
The same goes for comments. Once you’re posted them somewhere, you’ve not only fixed them in a medium but also implicitly licensed that particular copy of your words to the owner of the wall where you wrote them for use as a wall decoration.