DARPA has released some tidbits of information in a briefing on how one might build a propulsion system that combines a Constant Volume Combustion (CVC) engine and a full-scale turbine engine to accelerate a hypersonic jet to multiple Mach.
It's called the Vulcan, and it's a demonstration program designed to power a full-scale reusable hypersonic cruise vehicle like the Falcon HTV-3X, and to do it by 2012. The key, according to the DARPA briefing, is to integrate a currently produced turbine engine like the F110-129 or the F119, with minimal modifications and a CVC (PDF).
The CVC, or "scramjet," will operate only at supersonic speeds. It's basically a constricted tube with few or no moving parts through which air is compressed at high speeds, with fuel combusted along the way. The exhaust comes out the nozzle faster than the air came in.
The turbojet engine is needed for runway takeoff and to push the plane from zero to Mach 4, where the CVC would accelerate it to Mach 6 and beyond. Ideally, both would share a common inlet and nozzle.
Bottom line for the military is a hypersonic jet capable of delivering 12,000 pounds of payload up to 9,000 nautical miles from the continental United States in less than two hours. It would also be used for reconnaissance, strike, and other critical national missions, like ferrying Tom Clancy characters to emergency meetings at the Kremlin.