Hey, very nice. Finally the GUI toolkit joins the 21st century! I've been using Eagle for a couple of years and the interface has always been my only major problem with it--on Linux it's a lot like running a Motif app from the early '90s. Now it looks and feels modern.
I actually just used the Linux installer and it (so far) appears to work just fine alongside my existing Eagle installation. I did install it in a different directory, though. I've just had them running simultaneously on different viewports. The stable Eagle, not surprisingly, doesn't want to load files saved from the beta.
Thanks for the heads-up; I've been waiting for the beta to get a bit more stable but haven't really been following it.