The same thing happened to me sometime in the last week. My computer was off for several days (christmas), and when I turned it on, I had an IP ban. Using proxy works, but it's a needless hassle. If I delete the cookies, everything is ok, until I click the login link. From then on all I get is 403 on all pages. Annoying to say the least is, that there doesn't seem to be any auto-unban timeout.
And I don't comment all that much. Last month it was a comment every three days or so.
It made me think about the value of commenting for me and I decided to stop posting and lurk. Most of the value for me is in the lurking anyway.
http://news.ycombinator.com/unban?ip=<ip address>
You may have sent requests too quickly or something.
I enjoy the site as a news aggregator, but this place is an echo chamber.