Apparently you don't follow the news. The Kindle is sold out for months, so obviously there's a demand for that product. Why wouldn't Apple want to get a piece of a burgeoning market, like they did years ago with the ipod?
As of november 23, 2008 they sold 240,000 kindles. That's not very much.
Also, Apple already has mobile devices. The most I can see them doing is providing an easy way to access ebooks on the iphone/touch. The only difference between the kindle and iphone ebook hardware-wise that matters is the fact that the kindle has one of those weird "ink" screens or whatever. No biggie.