Running rtorrent and a webserver on the same machine reveals the following strange entries in the webserver's log:

127.0.0.1 - - [21/Oct/2011:18:59:02 -0700] "GET /announce?info_hash=.... \
                   HTTP/1.1" 404 142 "-" "rtorrent/0.8.6/0.12.6"

This happens every few minutes or so. It's not causing any trouble, but why does rtorrent connect to localhost? Someone at #rtorrent pointed out that it may be a tracker, resolving to localhost. And indeed, going through the trackerlist a popular tracker decided to resolve some of their servers to localhost. And they even explain why:

   Because retarded ISPs and registrars like to hijack NXDOMAIN responses to display 
   ads and crap. Also, if you are using a service like OpenDNS, you'll get their generic
   error response instead.

Understandably :-)

Well, redirecting localhost seems weird, so for now I'm just adding the tracker in question to /etc/hosts and point it to an unused private IP address to get rid of these bogus log entries.