All platforms (on iOS 6): 87%
iPhone: 93%
iPad: 78%
iPod touch: 80%
I'm a bit surprised as to why % of iPads running iOS 6 is so low. I mean, IIRC ~150M iPads were sold, and only 12M of them were iPad 1. But I guess his app (Audiobooks) is not a good representative of what people do with their iPads, so I take his numbers (about the iPad) with a grain of salt.
I've purposely kept my iPad on 5.x because iOS 5 maps is the only iPad maps application that works well with public transit routing. iOS 6 maps doesn't do public transit at all, Google Maps and all the good third party transit routing apps are iPhone only.
Also a bunch of the new iOS 6 features (like Passbook, photo panorama, certain geofences, and the updated phone app) are not available at all on iPads. Siri (maybe?) and IPv6 over LTE are the only iOS 6 features I feel I am missing on the iPad so I am in no huge hurry to upgrade.
I work on a pretty darned popular iOS app - these numbers are representative of what we've seen here also. iOS6 uptake on iPad lags substantially behind iPhone.
I can speculate on the reasoning but it's still a mystery. The sales of iPad 1's (which aren't compatible with iOS6) definitely contribute to the low numbers, but they do not explain the whole story.