Common sense...
I would buy a secret high-altitude spy plane a lot sooner than a secret orbital spaceplane. NOTHING gets thrown into an orbital trajectory that doesn't get the attention of the military forces all over the planet. How long would they keep that under wraps for us?
If the Pentagon does have this capability, it's odd, since DARPA is currently paying the company I contract with to develop the capacity to throw small non-weapon payloads into orbit.