vCenter 5.1 introduces the new Single Sign On (SSO) service with its own database requirement.
The creation of this database is manual outside of the installer. A great .sql script for this was posted by @vDerekS here.
I then encountered another error "Error 29115: Cannot authenticate to DB."
The solution was to toggle DB->Properties->Security to "SQL Server and Windows Authentication mode:
Happy 5.1 upgrading!