After a lot of hardwork and effort, a neowin member has found the key to enabling DWM (Desktop Windows Manager, lately renamed from DCE) in Longhorn"s build 4074, which was leaked last night. Build 4074 is the build leaked from WinHEC, which Neowin is attending and has exclusive coverage.
It would appear that a little tinkering with Longhorn"s DLL files allows users to enable a few DWM effects. These include fancy drop shadows on windows, and nice longhorn style icons are noticable on the left of the title bars. Also, animation is visible when opening windows. TWiST2K, the member who authored the resource hacks, suspects that there are more hidden options which can be enabled with this build, including wndows transparency and (hopefully) all the effects from build 3718.
Keep checking back, we"ll keep you updated as more information arrives and better screenshots are available!
Update: Neowin member, michaeltal, has learned of an easier way to enable the same DWM effects in the same 4074 build of Longhorn. The method simply involves the execution of the file "sbctl.exe", located in C:/windows/i386. The arguments "start" and "stop" respectively control the enabling and disabling of DWM.