I did that with some many of my Vista/XP computers, not thinking that eventually I would have to buy licences for every single one them.. Didn't backup my programs prior to upgrading either. Don't make that mistake!
It's better safe than sorry, though I honestly think it would be alright to run the beta, when Microsoft releases it to a public basis, they've obviously made their best attempts to ensure the product is as stable as possible - otherwise they'd have millions of upset end-users who ignored the warnings. *Vista beta builds not included*
I wish they had publicly available betas for programs and services I actually use regularly Last thing I participated in on Connect was probably the MSE alpha. I really am expecting a beta announcement at BUILD though, which will probably be released only for participants/TechNet/MSDN folks first, but obviously at that point it will be publicly leaked anyway.