I haven't been able to find one.... Well, correction. I tried to use Airplay which isn't supported right now. There are third party apps that do it I just don't know what the are.
A search for Airplay will likely lead to more tips for you.
I too have been wondering this. I also have been trying to see if i can Airplay my movies from my iPhone to my friends iPad (because of the bigger screen), with no luck.
Most results i found from searching the net are apps that work on Jailbroken iPads/iPhones only. As my iPhone is still under warrenty i won't be jailbreaking it any time soon :\