I was watching Roger Blandford's SETI Talks and he mentioned in passing that relativistic jets appear to emit radiation at speeds greater than the speed of light, but that this is only an illusion.
http://www.youtube.com/watch?v=flDQEQ5b1pk

My questions are:
1) What was the (incorrect) speed that was calculated by observation?
2) What causes the illusion? I imagine it has something to do with the warping of spacetime, but I have no idea.

Thank you.