My personal view on Virgin Blue allowing a passenger to do the safety briefing - what if there were non-English speaking customers on the flight ???
They would not have understood the p.a., and may have picked up some slightly incorrect information from an untrained person standing in the aisle.
I enjoy a good laugh as much as anyone else, but the safety briefing is NOT a laughing matter - it is there to help save lives, and I judge any airline on it's commitment to safety by the way it performs safety procedures.
By the same token, would a customer be allowed to land their plane - after all, there would be someone sitting next to them who was fully trained in doing it !!!!!!!!!