We want to be able to trim down history to say, the last three months worth of changesets.
This is not even theoretically possible. SVN is a pack-rat.
It is planned to be implemented (the OpenSolaris folks need it) but not yet.
Git 1.5 includes rudimentary support for shallow clones. Shallow clones are somewhat limited, though further development may remove some of these limitations. They can not be pushed from and can not be cloned themselves.
Usage is via:
git clone --depth x REPO
Where x is some revision depth.