eventually... probably...
I think tablets and ultra-thin notebooks (e.g. macbook air) will definitely replace MOST but not ALL computers as the years go by. Especially for "normal" people, i.e. people who are not power users.
Think about, what does the average person do on a computer? Internet, email... maybe some photo and video editing... some simple documents... You can do all of that on an ipad. Of course, these functions are limited compared to a real computer, but they are still quite robust, especially when you consider that the ipad is only on it's second generation. These abilities will only improve with time, as hardware (and hopefully software) improves and evolves. Think about it, a few years from now when you can get a quad core i7 equivalent in a tablet... you can do almost everything on it... almost...
Power users will likely need a real machine of course. When I run photoshop or Logic or try and write a 200 page biology thesis... I will not be using an ipad...!
But honestly, right now I think the biggest limitation on tablets (especially the ipad) is the software. In part due to ios, and in part due to syncing and itunes. Not having a REAL file system is a huge hassle if you want to use lots of documents and files while retaining the flexibility of a real computer. And flexibility is one of the true advantages of a real computer. If I have a photo that I want to edit with 5 different editing programs, that's easy to do on a computer. More restricted on an ipad (though not impossible). But, as a real (but trivial example), I put some comic book files on my ipad, hoping to test out the same books in multiple comic book reader apps... but oh wait, I can't! You need to transfer them individually into each app. There is no real file system... which is silly.
Syncing is the other problem though. What use is a tablet "computer" if it HAS to be tethered to another computer??? Especially since you can only tether it to ONE itunes account. Preposterous. And what happens if your main computer you use for syncing dies? You can't just connect to another computer and keep using the ipad, nope. It will erase your files! These sort of limitations (which are arbitrary and imposed by Apple, as opposed to a real limitation like a hardware issue) are the biggest hurdles tablets like the ipad will face when people consider if they want to buy one as a primary computer. And yes I know you can make back ups of your ipad and itunes library, but that's not a real solution. It's an inconvenient and time-consuming work around.
Hopefully these things will change... in fact, they will have to over time if the ipad is to remain successful. We should all re-visit this thread 10 years from now... the tablet market will be an interesting beast 10 years from now! It's already a game changer, quite frankly... but the market and especially the devices are all still in their infancy...
