Stratolaunch Systems. Their twin boom mothership will use six 747 engines. The 1.2M lb beast will, along with it's rocket system, be capable of delivering manned and unmanned orbital payloads weighing up to 13,500 lbs. They also allude to cargo transport missions with a 9,200 mile range. (via Space.com; video spotted via High Power Rocketry)
Scaled Composites + SpaceX...what a combo!