Some of Windows 7's features heavily protected in PDC build
It turns out that the build checks if you’re in an allowed domain and you’re not a temporary employee or vendor. So essentially the taskbar is supposed to be insiders only. The guy made a workaround which you can find at the page, but he warns that the reason the taskbar is locked is probably because it’s not finished: it’s a mishmash of old, new, and incomplete features. Instead of branching a PDC build and stressing the compile times, they just added in these pretty serious protections so that people wouldn’t have easy access to something that wasn’t ready to go.