Not exactly inspiring, but an alien enemy would be a pretty good way of uniting the planet towards a common goal.
Think of it: godless, drug-using communists in space!!!
http://en.wikipedia.org/wiki/The_Culture
[NB I've always suspected that Iain M. Banks is a Special Circumstances agent - installing an author to write "fiction" about themselves is exactly the kind of sneaky thing the Culture would do]. ;-)
I know what I would do given the choice!
I think that enemies you can see and anthropomorphise are more powerful, however, so I imagine a humanoid alien or terminator would be a better unifier than an invisible bacteria or global warming, even though all could be considered a common threat to humanity.
what I think should unite us is the understanding that this tiny blue dot in the middle of Milky Way is the only thing that keps us alive, so we need to strive to keep it so for as long as possible while searching for other ways to survive in space on a large scale.
And unfortunately, it's unlikely to become a popular political position either.
Imagine a chance to route defense spending to space research and exploration - it's the geek dream!