Just a followup, The FAA defines "powered-lift" as:

"A heavier-than-air aircraft capable of vertical takeoff, vertical landing, and low speed flight that depends principally on engine-driven lift devices or engine thrust for lift during these flight regimes and on nonrotating airfoil(s) for lift during horizontal flight."

I'd say that the Osprey falls into that category (as does the Bell Civil Tiltrotor, and the Harrier.)