with Ubuntu you have the option of upgrading every 6 months, every 12 months, or every 18 months without loosing support (other then in the time between the release at the 18 month mark and the time you move to it). this is ignoring the LTS releases.
I did not know that. How does it work out in practice - do they provide security support for two non-LTS releases back? Can you upgrade directly from release-2 to current, or do you have to do it in stages? The latter would be a major pain.
BTW, I am not sure that I agree that "regular" people upgrade every 12 months. The "regular" people who I know (e.g. my wife), if they used Linux at all, wouldn't want to upgrade ever. In practice they wouldn't be able to perform even a single upgrade anyway - almost none of my own Ubuntu upgrades have been completely trouble free. Perhaps a paid support contract from Canonical would help there, but $250/year may seem pricey...
(Of course the same problem applies to Windows too - when my wife installed Vista's SP1 on her own on her laptop her sound stopped working and the screen reset to 640x480, which actually prevents you from seeing the "OK" buttons of most dialogs :-)