iPhone version
I think Textastic is the best app that works on iOS 4 in this category.
Textastic iPhone version will be great.
Answer
Even if you design a new UI, would you really want to code on that screen?
Is anyone really using the iWork for iOS Apps to CREATE new Documents?
But in this case iCloud Support would be nice, so that all your settings are synced between your iPad and iPhone.
I submitted the iPhone version to Apple 10 days ago. I hope Apple approves it this week.
You can find the website with screenshots of the iPhone version of Textastic at http://www.textasticapp.com/iphone.html
There is also an updated manual available at http://www.textasticapp.com/iphone/manual/
The iPhone version of Textastic is available on the App Store: https://itunes.apple.com/us/app/textastic-code-editor-for-iphone/id550156166?mt=8
You can find the website with screenshots of the iPhone version of Textastic at http://www.textasticapp.com/iphone.html
There is also an updated manual available at http://www.textasticapp.com/iphone/manual/
Textastic 6 is a new universal app that works on both iPad and iPhone: https://geo.itunes.apple.com/us/app/id1049254261?ct=feedback&at=11lNQP&pt=15967&mt=8
Customer support service by UserEcho
Textastic 6 is a new universal app that works on both iPad and iPhone: https://geo.itunes.apple.com/us/app/id1049254261?ct=feedback&at=11lNQP&pt=15967&mt=8