I have the HP Mini with the 11.6" high definition screen. It's a very nice netbook, but still a netbook. I've had a few of these since they came out and this HP is the best. Performs well and does not have the "cheap, plastic" look like most of the others.
I'm going to buy an iPad. It will be a different type of experience. I do not see how one will replace the other. I know that over time I will actively use both and be satisfied with both of them. However since I'm getting the 3G iPad, I do believe the iPad will be the device that gets most of the use overall.