Ran into this issue today when upgrading a SQL 2008 Express instance up to Enterprise and trying to run Sharepoint Configuration Wizard. Apparently SharePoint 2010 Beta doesn't like SQL 2008 Enterprise without a few hotfixes...Those of which are still in beta themselves...Those can be found in the full text of the error I recieved as follows:
"SQL Server at XXX has an unsupported version 10.0.2531.0. The minimum required SQL versions are SQL Server 2005 SP3 CU3, version number: 9.00.4220.00, downloadable from “http://support.microsoft.com/kb/967909”, or SQL Server 2008 SP1 CU2, version number 1.00.2714.00, downloadable from “http://support.microsoft.com/kb/970315”.
Anywho, I'm downloading now and will install tomorrow. I'll update with my findings from there.
Good luck.