It does seem to be at least partially due to user agent, but I'm not
sure exactly what they're testing for. If I use the same user agent
that my installed firefox uses, I get the 403, even though firefox gets
in with the same user agent, but if I use a nasty message I won't show
here (hoping to leave that message in the logs, in case they ever look
at those logs), I get the page. I also get the page if I use an old
Internet Explorer user agent or a sidekick or Blackberry user agent,
but an Android one gets the 403. I've tried several of these multiple
times, and the results seem to be consistent, so I have no idea what's
going on. Just experiment with different ones, and something will get
the page.