I don't see them necessarily as being replacements for laptops, but for certain purposes they make sense, particularly if you're not typing much and doing more clicking.
For example, waiters taking orders, retail staff processing orders a-la the Apple Stores, mechanics running diagnostic software, warehouse managers doing stock checks, anyone wanting to watch or show clients media on a screen bigger than a phone.
Imagine apps like you get on an iPhone but for a device with more screen real estate.
In my opinion netbooks are too small for real keybaords and they only have limited use anyway due to their lower processing power and storage. So if you don't need a keyboard very often for whatever it is you're doing (perhaps surfing the web) these make more sense than netbooks imho.