According to Ostatic, Mark Shuttleworth, said that Ubuntu will likely be moving from its current six-month release schedule to daily updates which would make Ubuntu a rolling-release Linux distribution.
Today we have a six-month release cycle. [...] In an internet-oriented world, we need to be able to release something every day. That's an area we will put a lot of work into in the next five years. The small steps we are putting in to the Software Center today, they will go further and caster than people might have envisioned in the past.
- Mark Shuttleworth
However, the above quote is all Ostatic and The Register give us regarding this (and I for one won't believe it until I actually see the words "rolling release" used by Mark Shuttleworth).
Rolling release means you won't have to upgrade / do a clean install each time a new Ubuntu version is released to be able to use the latest software versions. As an example: Ubuntu 10.04 shipped with VLC 1.0.x and you cannot and will not be able to install VLC 1.1.x in Ubuntu 10.04 from the official repositories. The only way to install VLC 1.1.x in Ubuntu 10.04 is to use a PPA or upgrade to Ubuntu 10.10. That will not be the case with Ubuntu anymore (though this is not yet certain) - you will get updates for all your applications without having to install the latest Ubuntu version - you would already have it if you've kept updating.
Update: Rick Spencer says Ubuntu is not moving to a rolling-release. From his blog:
"Ubuntu is not changing to a rolling release. We are confident that our customers, partners, and the FLOSS ecosystem are well served by our current release cadence. What the article was probably referring to was the possibility of making it easier for developers to use cutting edge versions of certain software packages on Ubuntu.This is a wide-ranging project that we will continue to pursue through our normal planning processes."