Sources in Microsoft report that Microsoft missed the original internal release date of June 30 for Longhorn Beta 1. The sources say that Microsoft now plans to ship Longhorn Beta 1 in late July. The beta 2 release is still scheduled for November.
Internally, most Longhorn product groups have already shipped their beta 1 code to the Windows team for the final build. In fact Microsoft has just started prepping various 5200-series builds (post-beta 1) for internal rollout and testing.
The sources describe Longhorn Beta 1 as one of the most stable beta 1 OS releases in Microsoft history, although it will lack most of the UI niceties the company plans for future releases. Beta 2, I'm told, is going to be "incredible" and "far more impressive than people now realize." Well, they would say that, wouldn't they?
A note on build numbers: If you go to Help | About in Windows Explorer, you'll see the build number. Every night, Microsoft builds its OSes to incorporate the work done during the day. As such, the build number of work in development counts up by one each day.