I haven't verified this as I don't have Win8 beta to check, but this is supposedly the Reg Hack to start Win 8 with the "Classic" desktop:
The traditional UI is there - you need to change search for RPEnabled in the registry and change the values from 1 to 0. Most websites relating to this refer to only one instance - but I needed to change 3 or 4 instances. Then created another user named "Metro" with no changes. You can now log in to the Metro Interface or the Classic Windows interface.
