I just wanted to point you guys out to the following:
Issue: After attempting to upgrade to VirtualCenter 2.5 Update 3 using the VirtualCenter Update Wizard, and not the VMware Unified Installer, the VirtualCenter service may fail to start. The issue is caused by an incomplete update of the database information for VirtualCenter resulting in incorrect database versioning, which is verified, and required to be correct, by the VirtualCenter service upon startup.
I’m not gonna post the answer to this problem, cause the article might change over the next day and than this info will be outdated and misleading. So check this KB article for more info.