Thoughts: I also have not seen an Relational Database System that includes the notion of branching... though the concept itself seems quite useful IMHO.
I know Microsoft Team Foundation Server + Visual Studio 2010 (Premium or Ultimate editions only) has database development integration with source control via "Database Projects". I'm sure other version control systems have similar capability... but this doesn't answer your question for a lightweight or built-in DB version control system. (Therefore this is technicall NOT an answer - just some information that may help you resolve your base question).
Brief searching found some useful related articles but doesn't appear to exactly answer this request:
- RedGate SQL Source Control 3.0
- "Versioning Databases - Branching and Merging" - OdeToCode.com 2008
RedGate is a top tier DB tooling company that develops solutions for Microsoft SQL Server, Oracle, and probably other database solutions. The catch is that RedGate SQL Source Control 3.0 enables working with your version control system within SSMS (Microsoft SQL Server Management Studio) but it's not a DB VCS (version control system) on its own.
Excerpt from their website:
SQL Source Control is an add-in for SQL Server Management Studio that
lets you get your database into source control. SQL Source Control
therefore brings the change management and collaboration benefits of
source control to database development, without affecting your
workflow, or requiring new development processes. Note: SQL Source
Control is not a source control system; it allows you to store your
databases in your existing source control system.