http://www.psu.com/forums/Vita-and-P...a012985-p0.php
It has been today revealed that the PlayStation Vita and the PSP will be capable of communicating with each other, and even play with each other.

It's already known that the Vita can download and play PSP games, but what hasn't been clarified is if those games can be played cross-platform on PSP and Vita. Luckily, we now all know that our previously purchased PSP downloadable games can still be used in full functionality on the Vita.

Source: Siliconera