Heh, there's always that risk. Generally if the beta bricks your console, MS will replace it. (I've had that happen)I was going to sign up but in the agreement contract it says beta testers risk having their Xbox bricked...so I opted out...
Before the Beta gets to the public, it goes out to the xbox-team-only takehome group, and then the microsoft-only private beta group, and then the public group (With often a week or four in between - and sometimes the private and public beta groups get merged). There are enough consoles in the first two groups that any ship-stopper problems can usually be caught before they go public.
Sometimes it feels like every time I turn my console on at home, it takes an update...