I saw Brennan's query letter on her agent's site. She did not mention her fan fiction. She discussed it with her agent once publication was on the horizon (i.e., "Is this going to be a problem?") which is why she took down all her stories from all the HP archives.

Although the Internet buzz from her fans might seem to be significant for those already in fandom, I doubt it really had a major impact on sales.

Bloggers usually get published as a result of their blog being popular, and they end up writing a book that is basically similar in content to what they blog about.

I don't think anyone has ever been approached by an agent or publisher who said "Hey, your fan fiction is really good, what would you think of writing an original novel?" though.

Note to any agents and publishers who may be reading this: Feel free to approach me if you want to set a precedent! :D