I just built a demo system for presenting Commerce Server 2009 R2 CTP on top of SharePoint 2010 at the Microsoft MVP Summit. The experience was NOT fun, hampered by two major gotchas:

• SP2010 will not configure unless you are connected to the network. And the domain controller of the domain to which the machine belongs is accessible. The error will complain about configuring the database, but this is the real root cause.
• CS2009 R2 CTP – leaving authorization disabled caused – of all things – an authorization error. Configuring the XML files for Authorization Manager rectified the problem.

Many, many hours killed – if you run into these – hopefully they help!