ANT libs can also go into the user's `$HOME/.ant/lib` dir, but not sure if that's any better since the kegs are shared across users.

I can understand the cross-keg installation restriction, but isn't that essentially what's going on here (and probably worse because its version-specific and won't get uninstalled correctly)?
https://github.com/mxcl/homebrew/blob/master/Library/Formula/postgis.rb#L72