US Military GPS has access to a separate P(Y) signal on the L2 frequency that civilian GPS does not.

I'm not actually air force, but am a bit of an aviation buff with a private license. There would be backup nav systems in manned aircraft, but several popular modern munitions are GPS guided, and I would tend to think that modern battlespace information systems require it as well. Planes wouldn't fall out of the sky, but it would be a significant departure from the ideal state.

Descend until you can see the freeway and follow that is always popular!

Yeah, I was joking a little. Most aircraft would still work pretty well off of inertial data. However there are quite a few weapons systems that just plain won't work without GPS.

BTW, I'm not a pilot, but I work for a government aerospace contractor, so I have some experience in this.

Depending on the plane, there are cases where a full systems failure knocks out every means of navigation other than "Mark I Eyeball": http://www.defenseindustrydaily.com/f22-squadron-shot-down-b...