It is not in beta as far as I know and you won't have aceess to download
it. The only way I know of to have a build of Longhorn is if you were
at the PDC Conference or had an MSDN subscription and requested it
You can use a theme pack to see the visual of Longhorn if you use win xp.
Look in the google for a download url, in search inputbox put
Longhorn shell pack , theme pack or lhtrans6update.zip