Interesting survey purporting to show that the iPad as-an-eReader market share is increasing while the Kindle share is decreasing and that the two devices dominate the eReader market with Sony a very distant third:
iPad Closing in on Kindle in eReader Ownership
These results don't quite reflect the relative numbers of persons viewing the respective device forums here. Typically there are 5 - 8 times as many persons viewing Kindle versus iPad and 2 - 3 times as many viewing Sony.