Well, it looks like info about Longhorn at WinHEC 2005 is starting to make its way across the net. Some screenshots posted in the NeoWin Forums, displays information about the required builds that Longhorn can be upgraded from (5019 and up). Also, the minimum hard disk requirements is at least 10GB’s.
Someone else said that the preview build number is:
(6.0.5048.0 winmain_idx02.050401_0536) I wouldn’t put all my faith in it though.
The build also displays information the Setup Main Screen about "view Beta 1 release", which suggest its sort of being prepared for beta 1 which is said to be Summer.