This has to do with the digital signatures of certain drivers that aren’t upgraded quite properly. The typical error message you will see after upgrade is: “Windows cannot verify the digital signature for the drivers required for this device. A recent hardware or software change might have installed a file that is signed incorrectly or damaged, or that might be malicious software from an unknown source. (Code 52)”
This will not happen if you do a fresh install of Windows 7 Beta build 7000 64-bit. It’s a known and annoying bug, but it is being worked on for the upcoming builds.
To workaround the issue for now, press [F8] on boot and disable driver signature checking if you need access in the short term before a fix is available.