At yesterday's Ubuntu Technical Board meeting they discussed how long they will support non-LTS releases as well as the rolling release model for Ubuntu Linux.
It was decided at this meeting that non-LTS versions of Ubuntu will now only be supported by updates for a period of nine months. Up to now the non-LTS releases were maintained for eighteen months but Canonical has been wanting to shorten up this time or even do away with these interim releases in order to free up resources at the company.
This shortened maintenance/support window will affect Ubuntu 13.04 and all future non Long-Term Support releases.
It was also decided at the meeting that users will be able to continuously track Ubuntu development in a "rolling release" manner rather than needing to explicitly upgrade the system. The technical details to this are still being decided.
The IRC meeting log can be seen here