A website that you go to with a live JS edit/run cycle, saved on the server on the fly, EtherPad style.

An option to have it email the code to your iPad email address as a clickable link which is a javascript bookmarklet.

An option to have it save the javascript as a bookmarklet/link which you can on the home page.

This is a hack on the iPhone - encode a PDF into a Javascript bookmark and sync it to Safari's bookmarks (i.e. http://pimm.wordpress.com/2007/07/18/how-to-read-pdf-files-o... ).

Maybe if the server had a way to remember or reopen and tidy your code it would be closing in on PG's RFS#5.

You can't cron-job anything, and you can't work with any language other than Javascript unless you make a server side to->Javascript compiler for it. Another user suggested that there is now or soon to be inter-app file transfer, so depending on how that is implemented it might open other venues.