There is no official rolling release, all the supported Ubuntu derivative releases (Kubuntu, Xubuntu, Lubuntu, Edubuntu, Ubuntu Studio and Mythbuntu) are based on the Ubuntu release schedule, 6 months per released version and 1 LTS version every 2 years.
There are many rolling releases with unstable Debian based, ie:
The closest that you will find from a rolling release supported by the Ubuntu base and using Ubuntu packages is +1, run it always on alpha or beta.
There is nothing wrong with assuming that Ubuntu +1 is something as a rolling release except for the fact that there is a EOL for the versions and once a version is released you have to force the system to use Ubuntu +1 again. It does not really fit the criteria for the name rolling release.
Its not a rolling release per se, but is the closest you will get.
Rolling releases (by definition) goes against what Ubuntu fights for every 6 months, basically before each release, packages are imported from Debian Unstable continuously and merged to the Ubuntu repos with modifications done for Ubuntu. A month before release, imports are frozen, and packagers then work to ensure that the frozen features interoperate well together.
Its a lot of work to be done and supply you with stable packages.
To move your system to the next release use the command
sudo update-manager -d, it will allow you to install the packages from the future release
You will receive constant updates for this until the version is really released in April 2012, after that you will be able to apply the command again and update to the next future release after a couple of days.
Please note that this is not the intent of Ubuntu and that (as mostly any rolling release) the packages might break your system. If you are not so familiar with recovering a broken system this is not really advisable. Having said that I have to explain that depending on the skills some Ubuntu users "live" on Ubuntu +1 (mostly bug catchers and developers, but they exist).
In the end its up to you: the unstable world of Ubuntu +1 or the stable comfortable sight of updated stable packages every 6 months.