I read alot of financial news sites and they are always talking about the "death of the PC". Well bitcoin mining is a great example of something you can't do on your cute little iPad or iPhone.
Your average (and by average, I mean greater than 99% of base) user will not be attempting to mine bitcoins, much like your average person is not out in the dirt with a pickaxe looking for gold.
The death of the full-sized form factor PC is definitely a possibility. We're in the midst of putting out a home media appliance that also functions as a complete PC (currently supports Ubuntu and Debian) and it's the size of a small block of cheese. When something that size can contain a quad core 1.7GHZ chip, 2GB of RAM, support 1080p video and use next to no power - you have to start wondering what the point in much larger (and more powerful) machines are. Remember, your average user isn't even a gamer, let alone a miner.