Perpetually Updateable Linux Distribution

Would anyone have any ideas which Linux distribution can be updated the easiest, as in 2 years later, I can run a command or something that will bring it up to the most recent release without having to be fully reinstalled. I will be working with Xen soon, and I know that is more friendly for that type of thing than something like OpenVZ or Virtuozzo could be. I say this after seeing the 2 releases per year idea that Fedora seems to think is fun, and how much work it has been for people to keep up to date. For hosting, its hard to deal with uprooting every so often and migrating to a new system just to keep up to date. I would love to hear some perspectives on this from anyone experienced in this area.