Waiting until after the "2012 prediction" is behind us would be best. It seems most people can't incorporate both the Mayan End of Days and climate change into their future plans and goals, so adding a third 'hoaxable' problem to Earth's future won't help.

With that said, I can see society accepting a killer asteroid over climate change - since the science for the latter is more abstract than "hey look at this asteroid coming right at us."

End of the World movies and books are overdone, so there wouldn't be a need to break the ice to the public through popular media. I think it would be safe to simply state the danger directly.

It seems fear would drive some people to insane things, but enough people to be driven to help the situation, ie. agreeing to help fund, through taxes, research and operation of a mission to divert a killer asteroid.