During an upgrade of vCenter 2.5 to 4.0 at a customer site I decided it would be a good idea to record the steps required. In this video you will see how to upgrade VMware vCenter 2.5 to vCenter 4.0 that uses a Microsoft SQL database. The steps are trivial and I have done more than a handful of these on customers production servers.
The most important thing is to follow the best practices, give your ODBC user db_owner privledge on the MSDB and vCenter database prior to any install or upgrade – then remove the privledge after the installer is complete. Also make sure that IIS or any other webserver is disabled.