WinInformant says, that the PDC's Pre-Beta Longhorn supplied to the attendees will have the new GUI component Aero removed:
What developers will receive at the PDC, however, is very interesting. For the past several weeks, the software giant has forked the code for Longhorn, developing a special PDC build that is separate and distinct from the main code fork. This build will include virtually every Longhorn technology except Aero, and will include a new, darker theme that supplants the lackluster Plex visual style seen in alpha Longhorn builds. Microsoft is currently struggling to complete this special PDC build in time for the show, which will be held in late October.