This good article at itbusiness.ca has a run down of someone who is describing how to replace your office PC with an iPad.
Their conclusion is this:
It’s clear the iPad is not a perfect replacement for a laptop (or desktop PC). The connectivity issues create hassles that will slow workplace adoption, and the constrained data exchange and poor cloud-apps compatibility will restrict iPad business usage to basic office productivity work and, via thin clients or where specialty iOS apps exists, to niche field-force usage such as for hospital staff, construction foremen, and the like.
In my opinions, just like netbooks, the feasibility of this all depends on your use of the PC. I believe that if you use a device for accessing the Web and are good with using Cloud based apps like Google docs or Zoho, then you can go along way with an iPad as your main PC. The more you depend on installed software, the less likely this becomes as an option. For now.