Linux Mint does this on their LMDE version. A rolling release will mean that Canonical must provide an easy way to rollback to previous package versions or to save snapshots of the user's machine before upgrading. You can't expect that an Ubuntu end-user to have the same technical knowledge as someone with experience with Debian, Gentoo or the BSDs to troubleshoot when suddenly their computer is not booting and that they need to change run levels or disable their display manager to figure out what's wrong with Xorg. Shit happens, and happens often.

Also, I've used Archlinux (now Debian sid recently with their systemd "upgrade" and other desicions that deviate away from their KISS principle) and haven't experienced any problem, then again I tend to run a maximalist[1] desktop. Stuff starts to break when you have a lot of applications installed with their own dependencies and their own team of people working on that application, I've always felt that the whole desktop environment on Linux is like building a tower with cubes, it's solid when you have a few applications, but as you start adding stuff on top, the whole things starts trembling away and it takes a really small problem to bring everything down.

If this is going to be done, they better provide ways to their users to easily recover from an upgrade gone wrong.

[1] - Maximailism is a better word: http://kmandla.wordpress.com/2010/05/05/maximalism-is-a-bett...