Just from a users perspective, this approach sucks. Ever tried to install a package on an older Ubuntu box only to find that the binary for this version has not been updated in over a year? Then you find out why: installing the (old versions) of shared libraries inevitably breaks installation of normal packages, and is usually followed by headaches and backtracking. I'm sweating just thinking about this!

GoboLinux for the win!

http://gobolinux.org/

Oh .. wait. Its dead. What a shame. Well, maybe its time to revive it - or at least, its concept of packages - so that we can solve this problem.