I would be quite surprised if all second gen devices and even first gen didnt get WP8. Just as Mango brought new hardware capabilities (FFC, gyro) that 1st gen devices did not have I expect the same would hold for WP8, newer devices would be compatible with the newer hardware capabilities (NFC, multi core, BT 4.0, HDMI) and be able to exploit them and older devices would get the software but be unable to use all the advanced features.
I expect all WP devices to date to get some flavor of WP8, just depends to what extent. From what we've seen so far from MS on their update philosophy I think the Lumia devices will be fine for WP8.