The resolution to my problem turned out to have nothing to do with
m0n0wall, and everything to do with outband NAT from my company's network.
(my m0n0wall is on my home network.)
Some NAT setups break PPTP, and this is one of them. When I tried
connecting from a coffeeshop down the way, it worked just fine. :)
<adam hirsch> Experience is not what happens to you;
<adam at baz dot org> it is what you do with what happens to you.
<http://web.baz.org/> -- Aldous Huxley