I was just notified that there is also a build of longhorn that is not yet released to the public that who's version string reads For testing purposes only. Build 4050.private/lab06_demo.031019-1809., Yet that is about 9 days old because it was compiled on October 19, 20003 where as the build that was handed out at the PDC was Evaluation copy. Build 4051.idx02.031001-1340 and compiled on the 1st of October. The Microsoft employee said that Aero is still under heavy construction and security, to prevent any form of leakage, as the rep himself had never really worked with it.
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View Screenshots and more info Here