First of all:
This article contains some information from Windows 8. Windows 8 is currently in beta and there’s no guarantee that the final product will behave the same or contain the same features. Nothing in Windows 8 is final and everything is subject to change.
Ok down to business; I assume this is a bug:
If you are using Group Policy Preferences to map your network drives, you may notice that the mapping fails when you log on to client with Windows 8 Consumer Preview (despite gpresult and event logs says things are fine). Some quick testing proves that this occur when you log on with a user account that has local administrator rights, and if you remove your user account from the local administrators group the drives will reappear next time you log in.
Why you would log in with an administrative account is another discussion so I’m just gonna go right ahead and say it for now: You should not! If you want more information why you shouldn’t, then stay tuned to this blog and I’ll tell you more another time.