AirServer - Bring AirPlay and AirTunes to your Mac
Allows you to airplay from your iPad to mac mini. Works exactly like apple tv. Even works with iTunes on mac. Unfortunately for now a lot of content is restricted. But I did just watch the Linkin Park iTunes festival concert on my 46" last nite! Awesome! iTunes movies play great, and so does air video (tho that's less needed on a mac mini since it already plays anything on the network).
Another trick. If you jailbreak your device you can install airvideoenabler and it will add video streaming to most apps.