I've seen this as a payload for military UAVs (Hunter); it's a 700kg aircraft, though.

I don't think you could put the right electronics on a small quadcopter, but the gas powered quadcopters are perfectly capable -- anything which can carry a DSLR.

The nice thing about antennas is they're super directional (if desired), so really all you need is altitude, not necessarily maneuverability -- a tethered balloon or just being on top of a building would work fine. For SIGINT, you probably want longer loiter times than a small UAV will give you; a small UAV with wifi/cell would mainly be for tracking a moving target, or killing a target which happens to be radiating on a specific frequency (i.e. find/kill a certain cellphone).

In general the military cares more about cellular signals than wifi, at least so far.