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
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.
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.