Jeremy wrote a great piece (again) on progressive enhancement, and how it's often misunderstood.
I think I understand why Ola reacted so strongly to the suggestion that offline functionality should be added as an enhancement. I’ve seen the same reaction when I’ve said that beautiful typography on the web is an enhancement. I think that when I say something is an enhancement, what people hear is that something is just an enhancement. It sounds belittling. That’s not my intention, but I can understand how it could come across like that. Perhaps this is one reason why some people have a real issue with the term “progressive enhancement”.
That in a nutshell is what makes progressive enhancement so essential. It's insurance. It's a failsafe. It just makes good business sense.
Also: is a shopping cart a web app or a web site?