Mm, Microsoft don't distribute Pre-Alpha builds to their large Beta Testing network. Generally the registered public beta testers, like myself, will get it through Microsoft at RC1 level, or maybe just before.
Builds that are showing up now are leaking from a smaller group of supposedly 'trusted' testers that are lucky enough to get to try these early builds.
As it stands though, WinFS is crap, the Longhorn Bar is a pathetic joke compared to free alternatives that have been around since Windows 2000, and the new rendering engine is SO slow. But then, given a two years from now release, technology will catch up and maybe make the new engine and the WinFS concept feasable.
Until then, long live XP ^_^