According to Paul Thurrott, Microsoft will make Longhorn pre-beta builds widely available in November for a small fee (I'm guessing similar to past Consumer Product Preview programs):

The 7000 PDC 2003 attendees will receive Longhorn build 4051 on Monday, so I'll provide screen-shot galleries and a full review as soon as possible. But don't worry about pirating the build. Microsoft will make it available online to one and all in November for a small fee, I'm told, and ship it to Microsoft Developer Network (MSDN) Universal customers in December. Hundreds of thousands of copies of this important pre-Beta 1 release will be floating around, and Microsoft wants your feedback, which will dictate the timeline for Beta 1 (due vaguely in the first half of 2004), the features that will change by the final release and, ultimately, when the final release occurs. According to my sources, the oft-delayed PDC build (it was originally due in early October but was held up because of a weird CD-ROM-drive incompatibility problem) is rock solid and quite usable, unlike the other alpha builds we've seen. And, as first reported in WinInfo Daily UPDATE, no, it doesn't include the Aero UI that the final Longhorn version will use.