One of the largest problems of current UAV ops that I see is the simple lack of ability to see and avoid ... other traffic.
A few Predators seem to have begun flying surveillance along our southern border, for DHS or someone like that.
No few pilots flying in American airspace are aware of how a Pred works, and what it can see, and how it sees. Not pleased, in general.
Most UAVs are hard to see.
Most UAV's can't "see" as a pilot does for visual separation.
Expect more midairs.
I seem to recall that at least one midair was recorded in Iraq between a fixed wing and a UAV (military, both of them), and also a midair between more than one helicopter and UAV(again, both military) but my memory is a bit shaky on that. Been a few years.
There are a wide variety of UAV's, from hand launched to things the size of a pred and larger. You fill the air with enough of them, close proximity is bound to crop up more often.
Only one party to the close call has ANY ability to See and Avoid.
That is the manned aircraft.