Well in addition to the color LCD, they do have speakers so Google could do audio ads.

Also, Google+ checkins when you walk into your house and G+ status updates when you adjust your temperature would increase the G+ 'active user' count for them.

Also, I could definitely see myself buying Lord of the Rings in the Google Books store and reading it on the Nest.

This could provide the ideal way of monetizing this platform in the future.