I don't have any theme set or anything else I can guess would be hitting www.perlmonks.org all the time. I installed livehttpheaders and now can't get the problem to manifest :(
Update: (Note: this is Firefox 1.0 on windows) I get this to happen consistently by entering a perlmonks.org address while not connected (which pops up a Windows "Choose a network connection" dialog box), connecting via a different (non-gui) process, hitting escape on the connection dialog. Then Firefox (while leaving the http://perlmonks.org... in the location field) actually fetches http://www.perlmonks.org....
That is to say, just blame Firefox.