from a purely technical standpoint, you CAN write CI/PHP/JavaScript/HTML on an iPad. All you need is a text editor. You won’t have all the IDE bells and whistles but it would be like using Notepad on windows. However, if you want to use iPad for mobility’s sake, then you won’t really get all that much done on it. For instance I have my laptop equipped with anything I need to be really mobile: IDE, Apache, PHP, and everything I need for whatever projects I’m working on at a given time, so that when I’m on a 8-hour flight from Detroit to Amsterdam I can keep working just as if I were in my office connected to my dev server. With my laptop I can be without an internet connection for weeks and still be able to get my development done. I don’t think you can get all that much done on an iPad. In my opinion the iPad is just another gadget for people who have a fascination with gadgets and have the disposable income to sustain such fetishes.
I can read my e-mail on the go on my smart phone, I can view/edit Office documents and pdfs on my smart phone, if I really can’t wait until i get to my laptop; I can do all that and a lot more on my phone, which I already pay almost $100/month for, what on Earth would I need an iPad for?
But hey! That’s just me.