It would be great if one of the new teams published a step-by-step run down of their car from conception to track. I know USF1 have released a few little bits and Lotus have shown us their basic concept. Unfortunatly secrecy in F1 means this will never happen.
I'd imagine, the basic packaging of components e.g. engine/fuel tank and driver probably come first, especially if the engine is from a third party (Cosworth). Then, front wing/nose and DDD probably take priority as they can produce a lot of downforce and in the case of the front wing, have a large influence downstream, and the in the case of the DDD, require good integration and design upstream.
From there on I imagine its form following function and detailing.
I've been having a go at designing something in Google Sketchup and its pretty much DDD and nose/front wing which takes up the most time.