Long-Term Support releases of Ubuntu Server have been supported for five years, but in what may be an attempt to lure more corporate users to the Ubuntu Linux desktop, Canonical will now be providing desktop support for a length of five years. This five-year desktop support plan will go into affect for Ubuntu 12.04 "Precise Pangolin", but the current Ubuntu 10.04 LTS support period will go unchanged.
Canonical also plans during this five year desktop period to provide new hardware support through SRUs (Stable Release Updates) and point releases for the first two years of the LTS cycle (i.e. until the next LTS release comes, Ubuntu 12.04 LTS will keep seeing new hardware support until Ubuntu 14.04 LTS). It will be interesting to see how they handle the greater hardware support during this period, particularly for the graphics drivers.
The support cycle for Ubuntu non-LTS releases or the server LTS build isn't changed by today's announcement. Canonical fired off a press release announcing this desktop support extension and also fired off an email to the ubuntu-devel list about the new strategy.
This will also be discussed in just over one week at the Ubuntu Developer Summit in Orlando (I'll be there).
On a related note, Mark Shuttleworth wrote yesterday about UDS and planning for Ubuntu 12.04. Key points are that Ubuntu Precise will be the fourth Ubuntu LTS release, a big focus is on carrier-grade cloud infrastructure and guest support, a Pangolin-worthy server release, and providing a pixel-perfect desktop.