Life's too short to not write coffeescript on the train. So I made a HTML5 offline web app for your iPhone that lets you (try to) do just that. Head over and add Instant to your iPhone home screen, then click on the "Run" button to see the example coffeescript go.
The "Run" screen is just a DIV (called #output) - there is a helper function called "log" that just appends whatever you give it to the #output HTML... so it kinda looks like a console - but it's not. It's just a DIV.
I'm addicted to these HTML5 offline games and web app things. Apple store be damned! If I wanna run code on my iComputerPhone, then I will, I tells ya! Errgh, anyhoo... all this app is doing is running your code through the the coffescript browser library then eval-ing the output. The text editor code I nicked from the HTML5 textile editor by Guillaume Bort hacked up a little bit and using the canvas-text project to draw text (the iPhone doesn't seem to like the fillText method otherwise).
Is unlikely. I just wrote this to know that, if I wanted, I could code on the toilet.