Gentoo can be a pain in the ass, but the occasions when I need package masking and using precise, specific library versions, there's nothing else that does it.

Ultimately, I think Homebrew will win out because:

(1) It is on github (http://redmonk.com/sogrady/2010/04/01/github/)

(2) The Homebrew DSL is far, far easier to work with than Gentoo ebuilds

(3) From what I've seen, you don't need root privs to develop Homebrew formula, but setting up fakeroot to develop Gentoo ebuilds is a pain.

Homebrew or not, VM'd Linux hosted under OSX is a sweet spot for what I'm working on.