The biggest thing I'd be using Diet Coda for would be fixing small issues on the go.
I hope this it it.
(serious question for the folks who seem to need to do development on an iPad)
I'm switching to a 13.3-inch Macbook Air as my main computer (with a 27-inch external display). We'll see if this changes my view on not bringing my laptop places. Still, the iPad is much lighter than an Air and gets incredible battery life.
I might even be out at a coffee shop on the weekend and realize that there is something I want to fix on a website. If I'm out at a coffee shop, relaxing, reading some stuff, I almost never have a laptop with me.
When you get an iPad you realize laptops have instantly become less useful. iPads fit into a tiny bag, they have crazy battery life, and they can do many things as well as a desktop. It is a no brainer to carry them with you instead of a laptop, as they do 90% of the things you need a computer for very well.
Having a nice text editor/FTP program would be greatly welcomed.
But sometimes stuff comes up.
I have also begun to find the iPad a pretty nice environment for writing on. I have yet to find a program that I like to program in, however.