Relevant: http://david-smith.org/iosversionstats
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.