Panic released Coda 2 (AppStore) and Diet Coda (AppStore) recently. Coda 2 will replace Coda 1 for me, it's my cowboy coding (live on the server) tool and nothing is even close for that task. Probably more intriguing is Diet Coda though.
On my ride to Seattle I left the laptop (13" MBP) since carrying it for the 400km round trip was just too much. I could do everything but edit code. Sure it was unlikely that I'd need to do it but if some sort of emergency situation was going to come up, you can figure it would happen when I was away. Theoretically Diet Coda will allow me to do emergency coding. I purchased it but haven't had time to set it up yet so I can't speak from personal experience but there is a good review out.
And if you want some other ideas on turning your iPad in to a productive workhorse Matt Gemmell has a good list of productivity apps for the iPad.
I'm sure the day will come when we can walk around with an iPad like device and get all of our tasks done. My shoulders are certainly looking forward to that day.