When a tablet PC has the full unrestricted (Read: Not watered down versions) power of that years current generation high end hardware, with an OS and accessories of my choice (keyboards, gaming pads, mice, HDMI for external monitors etc..), then and only then will I say desktop PC's are dead or fading.
Until then, tablets are great for the segments they fill, but they can't and won't replace desktops or even laptops anytime soon for the upper markets, esp enthusiasts and a lot of gamers. I could see maybe laptops doing the job before tablets are able to.