For anyone else having issues setting up a VPN connection on Snow Leopard: if you are getting an “incorrect shared secret” error, quit wasting your time, and download Shimo, a VPN client for OSX. Haven’t paid for it yet, but will likely end up doing so, since it’s the only one of a half-dozen or so that I tried that not only worked, but wasn’t horribly designed.
I suspect my issue may have been related to a the VPN shared secret being too long, though that is mostly speculation. There was mention in a forum somewhere that OSX Lion silently truncates shared secret keys longer than 63 characters. Yay.