I just finished calibrating my iPad with the SpyderGallery app. Quick, painless and it made a visible difference. By doing side by side comparison to my other calibrated monitors, the images displayed on the iPad via the SpyderGallery app do appear more consistent to what I see on my monitors. Given that both the iPad app and the desktop server app that goes with it are free, kudos to datacolor!
Of course, the Spyder 3 is not free, but I had one in my gear box already
I stopped using the Spyder in favor of the ColorMunki, but this app gave me a chance to get some more use out of it!