The SR-71 is probably too small to carry a space-faring rocket, but the concept is reasonable: http://en.wikipedia.org/wiki/Air_launch_to_orbit#Supersonic_...

This splits combined-cycle propulsion into multiple vehicles: air-breathing propulsion for the aircraft, chemical rocket for the spacecraft, thereby improving the efficiency of the whole launch.

Not for orbital flight, but NASA did use one as a testbed for rocket engines, such as those for the NASP.