Well..I don't know about iPad. But there's an App on the Android market called Terminal IDE that lets you do programming using Vim right inside your device. It even come bundled with Android SDK.
"Yo dawg, I heard you like to program Android. So I put an IDE in your Android so you can program Android while you're using Android!"
Why don't you write an offline Javascript app to do it? Use one of the javascript syntax highlighters for the editor, the localstorage api to save, eval() to run, etc.
There's an iPod/iPhone app called jsAnywhere that's free and I like it. You can create projects, have an HTML, CSS and JS file, and run it. Might not have an iPad version.
While I don't have Webstorm, IntelliJ IDEA Community Edition consumes 250,000 kb with an empty project open while Netbeans consumes a bit under 400,000 kb, both without any sort of tweaking -- that's not that great of a difference to me. Vim, for instance, would be light.
Have you people even tried VS2010 for js? That thing actually executes your js in real-time so it knows the exact type of variables even after re-assignment and can offer type info/auto-complete that is 100% accurate.