One of the most destabilising factors of the introduction of drones to military arsenals is ability cross boundaries relatively easily without provoking an armed response.
Because drones can fly across boundaries with relatively little risk, there have been many border violations but few repercussions. For example in July, 2011, Lebanon complained to the UN that Israel had repeated violated its frontier, with overflights by Heron drones. It isn’t clear from the reports why Lebanon did not attack the drones, but it may have been a choice to not respond based on the politics of the situation. Or it may have been that the Lebanese military was physically unable to detect the drones quickly and mount an attack.
In 2009 drone incursions into Venezuela from Colombia provoked a war of words between the two countries.
Pakistan has often complained about American drone attacks on Pakistani territory (though it is widely believed that elements of the Pakistani administration cooperate with American drone programme covertly). Regardless, it is unlikely that Pakistan could militarily interfere with drones in the air, because American apparently drones apparently fly higher than the effective altitude of Pakistan’s F-16 fighter fleet.
There have been other violations of sovereignty, notably by Georgia in the brush war with Russia a couple of years ago.
An apparently Azeri drone of Israeli origin was shot down by the Armenians in September, 2011. The Armenians complained that the drones were hard to shoot down, in part because their ‘composite’ material (non metallic) make them hard to detect by radar.
There may be other unreported overflights occurring on a widespread basis, over many world frontiers.
Because the risk of quick detection is relative low, the options for response limited, and the penalty of being attacked quite low (no risk of death to a pilot, cost of thousands or millions, not hundreds of millions) there is an incentive to use drones to violate airspace in situations where it would be very risky to fly a jet.
Furthermore, because drones are normally considered to be much less dangerous than a military jet, there may be a tendency for drone incursions not to provoke so large a response. This may change with the introduction of ever larger drones, and the extension of remote control and autonomy to an ever larger range of vehicles in the air, land, and sea. Certainly there are now drones capable of carrying very large weapons including nuclear weapons.