Here is an amusing tale of how a military drone was hijacked and forced to land at what it believed to be it's home airfield. Drones like this havn't been around for very long, and their guidance systems are presumably quite simple - similar to the growing number of DIY drones based upon Arduino and similar microcontrollers or DSPs - so there is probably lots of low hanging fruit in terms of security vulnerabilities.
As I supposed many years ago there will now ensue an evolution of telerobot and counter-telerobot technologies. The most obvious thing which could be done to avoid denial of GPS or fake GPS scenarios would be to add visual localization. This could be as simple as dead reckoning based upon optical flow, similar to the method by which an optical mouse works. More accurate visual localization would combine an internal map with standard planar object recognition techniques based upon things like SIFT or SURF features in order to have the drone recognise certain locations and dead reckon between them. Those bag of words-like techniques can scale to thousands of objects, so there could be many landmark locations upon which the drone could localize. Of course, this assumes that the drone is able to see features on the ground, but if they're primarily used for surveillance then that's likely to be true most of the time anyway.
Sensor fusion between GPS and visual localization/odometry is the most likely solution. Also, this isn't only a problem which applies to military drones. In future I expect that most civilian aircraft will be flown in the same way, without human pilots, so increasing the robustness of the navigation is a worthwhile goal.