You can install it from here, when you first go to the page click "I want to be a labs tester", then visit the link again and you'll be able to download it.
One of it's coolest features I've seen so far (I've used it for 3mins!) is that you browse the videos online like normal, but then instead of playing the video you click the download button which talks to the AIR app and starts downloading the video in the AIR application. You can then watch the video for up to 30 days after you've downloaded it online or offline. They've used a Flash Media Rights Management Server to DRM-Protect content so you shouldn't be able to get to the video file. Pretty cool stuff.
You can find more info here: http://www.ashorten.com/2008/12/19/bbc-launches-iplayer-desktop-on-adobe-air/