Propelling a warhead of that size at those speeds poses serious technological challenges and Darpa estimates it will take more than 20 years to develop.
Over the next seven years, meanwhile, the US air force and Darpa will develop a cheaper "global reach" weapons system relying on expendable rocket boosters, known as small launch vehicles (SLV) that would take a warhead into space and drop it over its target.
In US defence jargon, the warhead is known as a Com mon Aero Vehicle (Cav), an unpowered bomb which would be guided on to its target as it plummeted to earth at high and accelerating velocity.
The Cav could carry 1,000 lbs of explosives but at those speeds explosives may not be necessary. A simple titanium rod would be able to penetrate 70 feet of solid rock and the shock wave would have enormous destructive force. It could be used against deeply buried bunkers, the sort of target the air force is looking for new ways to attack.
Jane's Defence Weekly reported that the first Cav flight demonstration is provisionally scheduled by mid-2006, and the first SLV flight exercise would take place the next year. A test of the two systems combined would be carried out by late 2007. A prototype demonstrating HCV technology would be tested in 2009. SLV rockets will also give the air force a cheap and flexible means to launch military satellites at short notice, within weeks, days or even hours of a crisis developing. The SLV-Cav combination, according to the Darpa document, "will provide a near-term (approximately 2010) operational capability for prompt global strike from Consus (the continental US) while also enabling future development of a reusable HCV for the far-term (approximately 2025)". The range of this weapon is unclear.
If you want to go deeper into space, the PAC-3 antiballistic missile also doesn't carry explosives and relies on a direct hit. You won't find out if they are on booster rockets, though. We should assume they are. Look it up!