As of just a few moments ago there is content on the Windows Vista site which announces the limited availability of Beta 1. The software itself is being released to a limited number of official beta testers and hence you can't browse there and start pulling down a version to play with yourself - unless you are one of the lucky minority.
There is good news though. The site is now populated with factual information about Windows Vista including some details of the security features. You can also get hold of Windows Vista Beta 1 in September if you are a TechNet Plus subscriber together with an extensive range of Microsoft software along the lines of the MSDN subscription. I'm not a fan of pitching product to you via this blog but I'd say it's worth getting a subscription just to be one of the first to try out Windows Vista - click here for details.
Before you get too excited do note that Beta 1 has a subset of features, more will come at Beta 2.