Depends on why you buy a netbook
I see tablet/iPad devices fulfilling a different role to a netbook, in my life they wouldn't compete. But I'm not a typical user.
To me they would fill a whole between my netbook and my phone.
My laptop has grown too big to want to carry around much. But it needs that because too much on my life is on there.
My netbook is a handy device to take around when I can't be bothered to lug around the full laptop. I can easily use it in the office, computer room, coffee shop, bog... etc. But unless I was doing something specific it would end up in the living room with me.
The phone goes everywhere, Oh I could wind the clock back 20 years and not need to do this, but me world doesn't work like that any more.
I can surf the web and handle emails etc from there, but it isn't ideal. The screen is a bit cramped and I'm rapidly becoming an old git and can't see it that easily for too long.
So there is room for something to fit between the two.
On evening where I'm stuck at a desk because of the back ground flow of email is too high to deal with on the phone, then a tablet might allow better monitoring from the sofa.
For those who live every minute of their lives in facespace or chat rooms, then they might find a tablet a better fit than a netbook, they don't need a full PC and so the tablet might be a superior device for them
But you can pry my net book out of my cold dead fingers when I'm gone. For me at least they aren't a competitor.