I agree with the OP that over-promoting is annoying, and I agree with others who've said authors need to do some promotion. It's a delicate balance, and not all of us have a feel for it yet -- especially those of us with only one book out (so far).
Whenever a writer whose work I love responds to me via twitter, blog, email or forum, I'm thrilled. I enjoy interacting with authors, and as an author, I enjoy interacting with readers. If someone posts to my book's announcement thread that they've bought my book, I'm going to say thank you, because it's the polite thing to do. Hopefully that's not considered over-promotion.
