This link has probably been posted before on one of the AF447 threads, but I didn't find it in a quick search.
http://pbma.nasa.gov/docs/public/pbm...m/x31_sfcs.pdf Not an exact model for an AF447 loss of course but it shows how dynamic instability can be caused in a FBW aircraft.
Do the PRIMs adjust control system gains based on airspeed? It would seem logical that they would. This would be mostly relevant to flight prior to the first ACARS message. Wouldn't control gains increase at lower speeds to compensate for decreased control effectiveness at lower dynamic pressure?