Official word: Mum. Google never comments on this.

Unofficial words:

Nexus S is expected to be one of the first old devices to get the upgrade straight from google (hopefully around holiday season). Samsung won't be involved so no worries.

Nexus One ... might get along with Nexus S ... unsure but as with other devices with unlockable bootloader, we expect the community to chip in (Icecream Sandwich will be open sourced in a month or two ... unlike honeycomb).

For non Nexii devices, manufacturers can pick from google even before the sources are released to the community.

How well/quickly they take care of ex-customers depends on their ideology. Not providing upgrades can help boost short-term sale of new devices. Providing upgrades will help gain customer confidence/loyalty in their brand in the long term.

But marketing and sales teams are filled with short-term opportunist pricks, who care less about their company's brand and more about the current quarter's bonus.