While I disagree with those that say the PC is dead / will be dead soon, it really comes down to computing power, connectivity, ease of use, and screen real estate. Depending on your application, for most users, a smart phone is too small to do much other than check email, play Fruit Ninja, and tick people off on Facebook. Actual decent web browsing, movie watching, paper writing etc... becomes a reality with a tablet, especially when you throw a decently usable case and keyboard in the mix... (On screen keyboards do have a huge suck factor...).
For serious computing tasks, graphics / video / audio editing / processing, well... tablets have a LONG way to go... Of course having said THAT, I would put my cheapie Pengpod 1000 up against my old top of the line AMD Athlon 1400 512MB ATA100 hard drives PC from the late 90s / early 2000s.... any day of the week, without any hesitation... But compared to my newer octo core 32GB SSD box.... Forget it...