There aren't a lot of options for writing code on your iPhone, so picking the best option wasn't all that difficult.  Nonetheless, Nebulous Notes wins out thanks to its better-than-average Dropbox-syncing abilities, handy time-saving features, and ability to serve as more than just your programming text editor. 

Save with CR+LF line endings for compatibility with Notepad.exe and other editors

"Insert Tab" key (multi-line tabbing and un-tabbing supported)

Plenty of mono-spaced font options

Create macros and add them to the toolbar, allowing for quick-access text substitutions

Can print via AirPrint or to a PDF file

Coding-friendly themes

Compatible with iOS 3.0 and works with even the first-generation iPhone

Where It Excels

Nebulous Notes is great because it's not only a syncing text editor for your coding needs, but for virtually any other use for plain text.  You're not restricted to coding, but it's exceptionally qualified for the job nonetheless.  While you might think that an iPhone or iPad isn't the best place to write code (even when syncing a Bluetooth keyboard)—and you'd probably be right—Nebulous Notes makes a big effort to take the pain out of writing code on your touchscreen device.  It supports TextExpander, so if you have that installed you can use your snippets within the app.  Additionally, there's a built-in macro system that you can use for quickly executing tedious keystrokes.  Those macros can be stored in the toolbar for easy access as well. 

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In addition to a bunch of great features, the $3 price tag gets you both the iPhone/iPod touch version of the app as well as a copy for the iPad.  When we originally looked at Nebulous Notes for the iPhone, we liked it a lot, but like any iOS app geared towards writing anything it excels on the iPad's larger screen.  While writing code may not be the first thing you want to do on an iDevice, Nebulous Notes makes it about as painless as possible. 

Where It Falls Short

It would be nice to be able to turn on and off a coding mode so you wouldn't have to deal with autocorrect issues while typing.  Additionally, syntax highlighting would be really nice.  Line numbering would be helpful, too (although, oddly, it is when you print but not when you're writing).  Also, sometimes the full screen and text options buttons get hidden by the keyboard and it's difficult to get them back. 

The Competition

There isn't a ton of competition for iPhone-based programming text editors, but there are a few options worth looking at.  For starters, CodeToGo ($3) lets you write code (in many, many supported languages) and actually test it within the app.  Nimbus Source Code Editor ($10) is pricey but supports multiple storage options, like FTP and WebDAV, in addition to Dropbox.  It offers line numbering, too.  Another option is to just use an SSH app (like Prompt) to access your web server and then edit your code via the command line in emacs (or whatever you prefer). 

Lifehacker's App Directory is a new and growing directory of recommendations for the best applications and tools in a number of given categories.