I'm guessing that it will be out around PDC 2008, which is in October. I'll probobly install it then, as they say that it's very mature, and I'd be interested to see how much faster it runs, compared to Vista, on my laptop.
Edit: according to Gizmodo, it seems like it will be December 2008 instead: http://gizmodo.com/5048501/windows-7-beta-1-coming-in-december-probably