App Store - Remote HD
This is pretty slick. A little steep at eight bucks, but it's full functional and pretty solid. This is actually a screenshot from my iPhone, connected to my mini:
It has a 3G option for it as well, so you can actually connect to it over the internet using your normal data connection in addition to wifi. It can work as a plain old remote control as well, but RDP in your pocket is much cooler.
I got splashtop hd for this as it was cheaper, but I use it on my iPad, it works ok but I thinkim gonna be looking for a better option soon as its a little slow at times.