Boxee recently released a brand new version of their beta software, but unfortunately, that build didn't happen to work on the Apple TV. That's now been fixed, however, and you can now get the USB flash drive creator app that will give you a patchstick to install the latest version on that Apple box hooked up to your television. Here's a quick FAQ on the whole process if you need a little help with it, but it sounds pretty straightforward: set up the patchstick, power up the AppleTV with patchstick installed, and enjoy. And even if you have alpha currently installed, you can just put the stick in and upgrade to the beta, no worries.
That's some very impressive and quick work by a bunch of people who aren't obligated to do this at all (there are a few XBMC devs on the team as well, and the patchstick will allow you to install XBMC also, if you'd like).