I ran into an interesting problem today which prevented me from installing Exchange 2010 onto 1 out of 4 Hyper-V guest OSes I was setting up. The problem was that the host couldn't find what site it belonged to for some reason. nltest /dsgetsite would return an error "DSGetSiteName failed: Status = 1919 0x77f ERROR_NO_SITENAME" and therefore Exchange setup couldn't pass the prereq checks.
This issue isn't just an E2K10 issue; it goes all the way back to Exchange 2000. See the following KB article for more details: http://support.microsoft.com/kb/264001
Checking the configuration of the OS didn't yield any red flags, and in fact the OS was built and configured from the same sysprep image as the other 3 that were working. IP settings were correct and there was only one virtual NIC assigned to the guest. All other domain related nltest queries worked fine, and the Site settings in AD Sites and Services were all correct.
Before wiping the VM and starting from scratch, I decided to try one more thing. I decided to uninstall the NIC from Device Manager, shut down the OS, use Hyper-V Manager to remove the virtual NIC, and then add a brand new one. It worked! When the OS came up it found the new NIC, and after configuring it with the same settings as the previous incarnation the nltest /dsgetsite command came back with the expected result - my site name!
So if you run into the same issue installing Exchange 2007 or the 2010 beta onto a Hyper-V virtual machine, try removing the NIC and reinstalling - it did the trick in my case. I've only seen this on one VM out of dozens I've installed Exchange on, but I didn't find anyone else via Google that had run into the same issue, so hopefully this post will help somebody out. Cheers!