I think it *could* be ok for content creation, but only if you're willing to accept some pretty big compromises.
First point: there will be 2 kinds of win8 tablets. The arm ones, which will be like the iPad, thin and light with good battery life, and the intel/amd ones that will be a bit thicker and heavier, and possibly have worse battery life. The serious content creation apps all run on intel/amd, there's unlikely to be much that runs on arm initially. So you'll need one of the 'big' tablets.
Next, you're going to need a stylus. Again all the creativity apps are built for standard windows, not metro. Meaning tiny buttons, scroll bars, general incompatibility with fingers.
Finally, performance is likely to be poor. The intel/amd CPUs that will go into tablets will be very low power, and they'll be nowhere near the speed you'd get in a desktop. Most of the software is built for the desktop, and expects a decent CPU. It's going to run slow in a lot of cases.
Of course all that will change if/when companies start writing apps for metro, but I suspect you'll see those apps on the iPad first. If they're going to rewrite their apps for arm + touchscreen, would they write it for windows 8 which is fresh and untested, or would they write it for the iPad with perhaps 80,000,000 active devices by the time win8 tablets are released?