See the newer announcement at the top of the Windows Live OneCare general forum titled:
Microsoft Security Essentials (Morro) BetaThe limited public beta began Tuesday June 23rd and many of us here are trying it out. It's currently closed since the number of participants has been reached, but might re-open again.
There are links for additonal info including the beta itself and the new forums where the current beta build are being discussed.
Rob