Months ago, the developers presented in the open the rolling-release concept for Ubuntu, essentially, launching an open-for-public-feedback debate on if, how, could-bes, probably, etc, related to Ubuntu as a rolling release.
As a consequence, the development version of Ubuntu (like today's Raring Ringtail) is to become a rolling release (probably in the near future), while the support for standard Ubuntu releases have been decided to a 9 months period of time.