Mixing CTP and Longhorn
Someone posted a method by which the Community Technical Preview edition of VS.NET could be installed into the WinHEC build of Longhorn. He noticed no problems. It involved removing a condition within the MSI installer.
Here's why Microsoft probably didn't allow the CTP to be installed. Longhorn's version of the CLR is several months old than that of the community preview.
Longhorn use version 2.0.31113.25 of the CLR, whereas the CTP is a much more recent build, 2.0.40301.9. The third number is the build number. It likely indicates the day the build was made (under the format YMMDD). The last number most likely reflects the number of attempts made to stabilize that build. That Longhorn's 25 is almost triple that of the CTP's 9 is a good indication of how much more stable Longhorn's earlier version of the CLR is compared to the CTP. The PDC build used build 30703, which had been prepared for months before PDC.
My advice is to save some grief and avoid mixing the two.