Assuming all households were once candidates for the Kindle, the iPad has just taken 90% out of the book reading market leaving just 10% - the serious book readers to buy the Kindle. Amazon CEO Jeff Bezos explains in fact the best way to compete against the iPad is with a more-focused approach. The Wall Street Journal has more and points out quite accurately that the Kindle service could potentially still do very well on the iPad.
Last night in fact I came dangerously close to buying a book on the iPad and agonized over buying it via iTunes to be locked into an Apple-only ecosystem or go Amazon so I could at least go cross platform in the future.
I did a quick search via the Apple bookstore and the volume didn't show up. Once I make my decision on which direction to go, I will be sure to share.