The one running Windows 8 RT won't quite be there as a full replacement (which is the one at the 500 price point), the only desktop app it'll be able to run is the Microsoft Office suite, it'll be limited to Metro apps past that. But it will have word processing capabilities at least.
When they come out with the Surface Pro that will be much closer to a full laptop replacement, as it will be running a full version of Windows 8 that can actually run any other computer program the same as a laptop running W8 would. However, that's not going to be released until January or so, and the price is probs going to be much higher.
Of course, once the app market is built up a bit more the RT becomes a bit more attractive of an option at that price point.