It's been pointed out on Schemepersist that savepoints (which are widely supported including SQLite) support the purposes of nested transactions, though a little tricky to use because there is no explicit commit.  But because they are named you can use them to support arbitrary depth of transactions. 

John Cowan          http://vrici.lojban.org/~cowan        xxxxxx@ccil.org
Business before pleasure, if not too bloomering long before.
        --Nicholas van Rijn