Apple with all its popular products that can be interfaced with iPad will gain significant market share by the end of 2010. See http://yovia.com/blogs/marketwise/2010/02/04/mixing-up-the-market-with-the-apple-ipad/ to see predictions about the iPad impact on the market.
The iPad has a "little" bit of overlap with the iPhone/iPod Touch and "Macbook" market. So I do see it might cannibalize a few sales. But, it is a product in it's own niche for the most part so they are opening up to a whole new category of sales... so all in all, I see a big increase in Apple's market share. When you combine that with the fact that they may get a cut of the AT&T "data" revenue, as well as increased App sales (and now, book sales) I think it bodes well for Apple's bottom line and see further increase of profits down the road. All in all, a good move for them even if the reception of the product is mediocre for now.