NASA got out of the space plane business a decade ago, after it became clear that expendable rockets were cheaper and more efficient. Last week, they decided to un-mothball one of their experimental space planes, the X-34, to check it out for a possible return to flight.
The X-34 was designed by Orbital Sciences and unveiled by NASA in 1999. It was supposed to launch itself into space at Mach 8, release a payload into orbit, and then land, all completely autonomously and for a tenth the cost of more conventional systems. It sounded great, but the whole thing was canceled in 2001 after (shockingly) it proved to be too expensive.
Since then, NASA has apparently been rather impressed with how well Virgin Galactic's space program is going: the company's system of using a carrier aircraft to boost a spacecraft a significant distance towards orbit is a fast, cheap and reliable way to do it. The old X-34s (there are two of them) have been taken out of storage, and engineers from Orbital Sciences are going clean off all the bird crap (seriously, check out the gallery) and figure out whether they're still spaceworthy.
If they are, the X-34s may start a test program that's basically the same as what Virgin Galactic does: a modified airplane would carry the X-34 up as high as possible, drop it, and then the space plane would fire up its own rocket motor to boost itself into orbit. After delivering its payload, it would glide back to Earth, where it could be turned around and sent right back up again. It makes a lot of sense to do it this way, we'll just have to see if a ten year old demonstrator vehicle is up to the challenge.