There isn't a "Beta" version of Longhorn.
Participants at the Professional Developers Conference received
a preview edition. Do a "Google" for Longhorn, There are many
websites dedicated to it's progress and screenshots, etc.
At the earliest, Longhorn won't enter Beta status until late this
year or early 2005. Recently, Microsoft trimmed back some of
the proposed features in Longhorn to help meet their tentative
delivery schedule. Besides that, a lot of Microsoft Longhorn
developers have been pulled into the XP Service Pack #2 prgm
to help get it out the door.
A good starting place for Longhorn info is found at
http://www.winsupersite.com/