I do some programming on mine by SSHing into my main machine via Prompt. And I do some writing on it, too. A Bluetooth keyboard makes these activities just as easy as on a full fledged computer and it is nice to be able to roll over in bed and turn on the iPad when inspiration strikes.
I actually love quick-prototyping music in garage band. Dream come true when it comes to bringing music from your head to life in a matter of minutes, well arranged, with nothing but iPad. It's the music sketch book I always craved.
I use Inkpad to sketch out vector graphics for my various projects. Then sync to my laptop via Dropbox and use inkscape to complete and the CLI tools to generate pngs. For simple things I can do all the vector stuff on the iPad.
I use penultimate to pretty much replace my note books to sketch out new ideas.
Task paper to plan projects.
Numbers for various little spread sheets I still have to keep, e.g. tax, bills.
Tab toolkit for well, tabs.
One day I will get better at guitar again and record something on Garage band. Until then I use Garage band and the Apogee jam to replace all the effects boxes I could never justify buying.
I've started drawing again thanks to Brushes.
Occasionally Textastic to review code on the go. I can fix small things if I have to or write up pseudo code that I fix up when I get to my main machine.
Some of this is not technically producing I suppose, but my iPad is now a very convenient part of my process.
As stated above, I use my iPad 1 for my full on-the-go computing needs. Granted, I'm a writer and editor, but still, that means I'm writing plain text or HTML posts, managing a team, responding to support emails in Mail or Desk.com's web app (and 50+ a day), all from an iPad.
And while this doesn't quite count as producing, a couple weeks ago I pulled up sheet music in Safari, put my iPad on a piano's book stand, and started playing a song that a friend had mentioned only minutes before. You'd never do this with a traditional computer, but today it doesn't even seem that odd or amazing. Sure, it was "consumption": I was reading sheet music from the iPad. But if that's not intellectually stimulating and productive, I'm not quite sure what is.
