Some of the other authors are making valid points, but it shouldn't discourage you IMO. It depends on what you're trying to do. What's true:
• iBooks Store is not going to be the first place people go look for books. Amazon is where people go and Amazon is not going to start offering iBook versions of author products. While the Kindle is not a direct competitor of the iPad, Kindle is most definitely a direct competitor with iBooks.
• The iPad is easily the most popular mobile computing device out there, HOWEVER... it's NOT the most popular eBook reader. Kindle is (by a huge margin). That's why the prior point is true. The ubiquity of the device for a particular task, defines where everyone goes to find their content.
So the question becomes how are you intending to sell your book? If you have this approach (which is traditional in the book writing business, but with a new twist):
1. Book idea
2. Write book (independently or under contract)
3. Put it in eBook format (used to be "have it printed")
4. Put eBook in a store, sit back and watch the sales while you do a few presentations/signings if you're lucky
...that process will get you nowhere fast if the iPad market is the only store you're using. There is not the volume of "search-driven-sales" on Apple's book ecosystem yet to make that work as a big revenue stream, unless you're a famous author and create a title which Apple helps you to advertise as "exclusively on iBooks". For the rest of us, Apple isn't going to spend a penny on promoting our works beyond the product pages.
If you have decided on iBooks as your primary sales mechanism, you can't rely on Apple to do the heavy lifting in terms of marketing. In this setup, the process should (ideally) look more like this:
1. Book idea
2. Build over many months a nice online presence, positioning yourself as expert
3. Write book
4. Put it in eBook format
5. Put it in iBooks store
6. Use your popular web site to promote the [heck] out of it.
The reality for many authors right now is, you have to distribute on multiple platforms and not rely on Apple. That ultimately means more grunt work preparing the same content in different ways. Hopefully in 4 or 5 years we won't have this problem in the sense that Apple's iBooks will have enough search volume that it's less of a big deal not to involve other retailers, etc.