According to Google, any Android 2.3+ device should be able to be updated to Ice Cream Sandwich:

We’ve just heard straight from Google’s Gabe Cohen that the Nexus S will definitely be getting ICS. In fact, both he and Matias Duarte think most Gingerbread devices will see an upgrade, saying: “Currently in the process for releasing Ice Cream Sandwich for Nexus S. Theoretically should work for any 2.3 device.” It’s hardly a surprise and there’s no specific word on timing just yet, but hopefully it won’t take long to move that vanilla Android device up to something with a breaded exterior. [...] We also asked about whether the Nexus One is getting some ICS love, but there’s no clear plan just yet.

Comment via this identi.ca conversation.

According to Google, any Android 2.3+ device should be able to be updated to Ice Cream Sandwich: We've just heard straight from Google's Gabe Cohen that the Nexus S will definitely be getting ICS. In fact, both he and Matias Duarte think most Gingerbread devices will see an upgrade, saying: "Currently in the process for ...