For those that don't know (some on this page don't, it seems), Arch, Gentoo, FreeBSD(-STABLE and -CURRENT), and others already do this.
So I really wonder how Ubuntu will pull that off (rolling release of customized packages?!)
The biggest difference with Ubuntu (and Debian) is that they vet the packages for a much longer period of time and have a whole lot of politicking and red tape involved in getting the packages even pushed into the unstable / proposed repository.