I googled this error and found a couple of things, but nothing specifically related to my situation. Finally, I just used my noggin and with my safety goggles on, I opened up the registry.
I searched for ".xlb" (without the quotes), and it came up with an entry in HKLM/Software/Classes/.xlb . I went up to "Edit" and then "Permissions," and found that somehow, the Administrators had lost control of that entry. So I clicked on "Full Control" for the Administrators, and clicked Ok. I went through the rest of the .x entries and did the same with them, just to make sure that would take care of the problem.
And wah-lah. Office 2007 Beta 2 is now uninstalled successfully. I hope that helps someone else. I wonder if MS knows about this bug...how could I submit it to them?