See the Penny Arcade post on the Microsoft Surface Pro. Gabe was able to go through the entire process of drawing and uploading a comic strip as or more easily than he would have done on his PC. This isn't the same as implementing an e-commerce site, but it's one professional workflow that you would've normally assumed would require a PC.
More workflows will be added for different fields and professions, and eventually the uses for a full-blown desktop PC will become rare. Sure, we may never develop full-blown webapps on our tablets, but we're probably only a few years away from the majority of our non-dev tasks being practical without a desktop PC.
Edit: Removed a repeated example
The Surface Pro has a Core i5 Intel chip, half the battery life, a couple of fans, is heavier, thicker and gets hot. It has a powered USB 3.0 and can drive a 2560x1440p monitor and has a Micro-SD slot, a full Wacom digitizer and a pressure sensitive pen. Not to mention a full user navigable filesystem Even the latest iPad has none of these features (or disadvantages).
It is more like a convertible truck/car hybrid though and Microsoft still calls it a PC.
The major downsides that I have experienced are 1) the terrible trackpad scrolling (no momentum and it's not sensitive enough - like all non-Apple trackpads), 2) Chrome doesn't work very well yet with touch, 3) the pen holder is unusable when charging and 4) there is no Fn lock, so I can't hit F1, F2, etc without pressing Fn.
No big show stoppers and I'm hoping all of these can be fixed with new drivers and software in the future.
While we're definitely moving in that direction, the workflow he used is still impossible on a normal tablet for the time right now. Sketchbook Pro exists on the iPad, but as far as I can tell there's no tablet version that works with real digitizers. For now, it remains in "Work you need a real OS for" category.