Microsoft has posted its schedule for Longhorn:

Dates:

Milestone 9 (platform complete) - 16th March 2005
Beta 1 - 25th May 2005
Beta 2 - 12th October 2005
Release Candidate 0 - 22nd February 2006
Release Candidate 1 - 12th April 2006
Release To Manufacturing - 24th May 2006

Hat tip: www.neowin.net.