You can sign up for the Quake-in-a-browser beta testing sessions, just here. It's your standard beta testing deal: the game is finished and now needs tweaking. Hopefully they won't have to keep patching for five years or whatever it was on the original game... It's not an open beta, so get those email addresses in the slot quick smart.
You know, Quake Live sure does make me hope for some kind of enormous Quake 3 revival where I can get to show off my madskillz again. I miss those glory days. Sigh.