We need someone with air force experience in here. It doesn't seem like they'd let them fly without knowing alternate ways of navigating.
Since this thread sprung up I've wondered what might be impacted by a GPS failure.
As far as BIS info, it's not something I've followed closely, but we're talking things like every plane, tank, and infantryman now (soon?) has gps and information on the location of all friendly forces is automatically shared so you cut down friendly fire.
Apparently caf has more info on milspec GPS than I do, I thought that it required the same main band as civilian, and just used additional military bands to increase precision, but apparently it can function fully independently (not really surprising when you think about it). At least that gives you a little info on some of the troubles of failure of military GPS, as unlikely as that is.
Using two frequencies makes the military receivers more robust, and also lets them do some clever ionospheric distortion cancellation that you can't do with one channel. (Basically they get the benefits of WAAS without actually needing to receive the WAAS signal.)
That said, neither the FAA or the military are going to tolerate any significant degradation of GPS, domestically or otherwise.
BTW, I'm not a pilot, but I work for a government aerospace contractor, so I have some experience in this.