There are actually some successful tablet PC systems out there but these are targeted to a specific demographic. The more powerful the hardware the more you pay. For example, I have an Asus Eee Slate EP121 which has an Intel Core i5 470UM (1.33GHz - Turbo Boost up to 1.86GHz) CPU, 64GB mSATA SSD (upgradeable) and 12.1" LED widescreen display among other features. It’s a great product but not intended for general consumption because few people are going to lineup to spend over ~$1100 for such a device.
Newer devises like the Samsung Series 7 tablet PC (Intel Sandy Bridge) command an equally lofty price and one would expect future Core i5 / Core i7 “Ivy Bridge” tablet PC systems to improve performance, increase energy efficiency and command a lofty price as well.
On the other hand, I’ve been hearing rumors of a quasi ~$200 Windows 8 ARM tablet using nVidia SoC. That could be widely popular with the masses.