^^ Good luck with that.
Anyways, I'm using 7 as my primary OS on two of my PCs, and have yet to have any problems.
It's a gamble, but if you're that anti-Vista, I'd say go for it. But that's just me.
Anyways, there's no way for MS to 'kill' your PC for having a downloaded copy, however; you can't activate it until the beta goes public, so until then you'll have activation nagging every couple hours.