In Duplicates existing functionality, Marco stresses that

With all of the discussion of the benefits and drawbacks of applying the iPhone OS to more general-purpose computing tasks, this is one aspect that’s easy to overlook at the beginning: software competition evaporates for anything already done by an Apple app.

I completely agree with his analysis that it will hinder future development on the iPad and iPhone OS. However looking at recent events I think Apple is taking a different stance on this.

First let's look at the state of bookstores available on the iPhone OS. There are already a bunch of book apps available in the app store, even a kindle app that takes you the the Kindle online bookstore to buy books. Following the above argument, now that Apple has an iBookstore, all existing bookstores are now duplicating an Apple app.

The thing to notice here as Gruber pointed out is that the iBookstore is actually just an app which you will need to download from the app store. It is not a native iPhone OS app. In fact they did the same thing for the entire iWork suite of apps.

This could mean that Apple is aware of crippling competition and is opening up the platform to allow competition. The iBookstore will compete with the Kindle app, Stanza and any other future book or magazine apps that are introduced. If Microsoft decided to release Office apps for the iPad, they will most likely be accepted by Apple. I believe that even in the near future, an app like mobile Firefox can make it into the app store.

At the end of the day, the ultimate goal is to sell as many iPads as possible. Restricting the duplication of functionality for the iPhone was okay, but the iPad will need a new set of rules and the availability of a multitude of different apps, even those competing directly with Apple.