According to the ReadMe, the RTM (V1) of DB Professional will again work with a local validation database SQL Server 2005 Express (as did CTP6 and before).
I can not test this since I don't have a box that I want to degrade anymore.
Apparently the DB Pro Team listened and reacted to the critisism on the forum regarding the requirements for CTP7. Thanks to the Team!
Alle

RTM accepts SQL Server 2005 Express for Local Validation Instance
CruzPedro