I agree with some of the other posters, I don't believe a smartphone will fully replace a PC. It might replace it for some people that only used the computer to check email and surf some of the more popular sites, but not everyone. People that love playing games (like WOW or EVE) or use programs like photoshop will still use a PC. Not to mention the big screens lol.
I do use my smartphone for portable monitoring of my sites, however, it's just easier to type on a real keyboard, and more accurate to use a mouse. Not only that, I don't have to keep worrying about battery life.