Chris, everyone is getting that same "not available" message from that site right now. I read somewhere that Microsoft will make the beta available again some time in the near future, just not sure when. So keep your eyes open on that one.
Myself, I was eager to try out this security solution and was disappointed when I found out that I can't get it because I'm in Canada. But based on reports I've read from others who've tested it out already, two key observations were made:
1. It's VERY fast.
2. It provides very good virus protection.
On the very fast front, some people mentioned that an entire scan of their machine was completed by the software in under 6 minutes, which is incredible. And in terms of virus detection, tests done by independent agencies rank this solution right up there with the likes of AVG and Avira (which I use).
Guess we'll have to see!
Yann