Git based package manager/installer for Linux

There's Exherbo, which uses multiple Git repositories to store its exheres (its term for build recipes similar to Gentoo's ebuilds or BSD's ports). It's still a pretty young distribution, though.

Update: Gentoo recently moved to Git for its package repository. However, I don't think it's set up yet to have normal users get updates via Git (although I believe it's planned to allow that).

Funtoo, a fork of Gentoo, uses a Git-based portage tree. Gentoo's ebuilds are periodically imported, so you still get all the advantages of its wonderful ebuild collection.