Aptana really is the premier JavaScript IDE out there with a variety of support for JavaScript, Ajax, DOM, Adobe Air and now the iPhone. Yep, Aptana has just released a beta of their new iPhone Eclipse plugin which will help you in building apps to work within the confines of the iPhone.
More details can be found here:
The cool thing is that the plugin will allow you to preview what your app will look like in a simulated iPhone page. Very cool.