From what I've read and seen most of the collision detection stuff that you see on youtube etc is using external sensors. In this TED Talk, Vijay Kumar mentioned that an overhead array of video cameras is what they use to feed their quads positional information about them and their surroundings http://www.ted.com/talks/vijay_kumar_robots_that_fly_and_cooperate.html
and a google search shows up different wireless networks where swarms of UAVs share their positional information with a ground station to ensure no two inhabit the same space (though that's not useful for obstacles that don't share that information).
Found this link on DIYdrones (http://diydrones.ning.com/
) about a study on how insects do it http://www.psicorp.com/library/SR-1450.pdf
(heavy going, but interesting stuff!)
For the most part it appears the most important sensor to link into your UAV for collision avoidance is still a human