I guess by some definitions, but in most companies, a product is in development when you have an agreed objective, specifications, plan and committed funds / resources to achieve the agreed plan.
Storm
Storm
I'm not sure if we've heard anything decisive about this, but I wouldn't be surprised if Bethesda started working on the engine for Skyrim during Fallout 3's development, since Fallout 3's engine was pretty much just Oblivion's. Then after Fallout 3's release, all the level designers and artists and so on started to work on those aspects of Skyrim's development.