I work on my iPad whenever possible (basically with the exception of dev and Skype), and I've seen a bunch of improvements.
First, it's absurdly easy to walk around working when you're working on an iPad. It's a walking desk without the desk part. Second, it motivated me to add a ton of Gmail filters, to keep the e-mail reading experience pristine. Third, I deleted Twitter from my iPad, so I have to go over to my standing desk to go on Twitter. Adding that tiny bit of friction reduces the time spent babbling to a manageable level.
Another thing I really like is reading code while on public transportation. It used to be that every minute I spent in traffic added negative stress; every minute I spend in traffic now is productive, because somebody else is driving, and I'm reading, writing e-mail, working on spreadsheets, or planning and outlining. (I have video games on there too, but on the whole it's a very productive experience.)
Also, I do sometimes use my iPod for Skype and my iPad for dev. I've detailed my iPad dev setup elsewhere; basically it's a ssh client, vim, and a tiny vimrc/terminfo hack which makes the Tab key act as an Escape key in vim. I also read code using an app called Source Code.