Thanks for your help Will. It didn't work, but maybe it will be fixed in a future release...
Are you using SilverStripe 2.3rcs or SilverStripe 2.2? We would love to get to the bottom of your issue rather then ignore it as I am sure other people will run into it.
just to note, I have the same problem... ss version 2.3 and forums version 0.1.2
going to update the function sapphire/core/control/Controller.php on line 367
it looks like there is a bug with the release.
and see how it responds but
aram's fix worked for me and there don't seem to be any further issues.