Thanks to sharp-eyed Steve Trimble at the Quad-A show, and his The Dew Line blog, we now know about Bell Helicopter's Hybrid Tandem Rotor (HTR) concept.
According to Steve's interview with Bell's exec VP for government programs Robert Kennedy, the HTR could achieve a forward speed of 225kt, compared with around 170kt for a conventional helicopter and more than 300kt for the V-22 tiltrotor.
Exactly how it works is not clear. To me, it looks like a compound - the wingtip engines providing thrust as well as driving the rotors. So in forward flight, the wing would offload the rotors and allow the machine to reach higher speed before retreating-blade stall sets in.
[UPDATE - Steve now tells me the engines purely drive the rotors, V-22-style, so that means the rotors have to provide all the thrust. Which makes the concept even more baffling to me.]
What I can't work out is quite what happens on takeoff and landing. Apparently the wing tilts by 25 degrees and it looks like the rotors tilt forward at the same time, to stay essentially flat. My best guess is tilting the wing up reduces download in the hover, and increases lift in STOL operation. In vertical flight most of the power would go to the rotors, which would work like a conventional helicopter's.
This is speculation, of course. I suppose I will just have to ask Bell...meanwhile I'm hoping there is a rotorhead out there who can tell us how it works.