You're on the right track. With the URL you need to use http://code.google.com/googleapps/appsscript/class_urlfetchapp.html in order to get it to call your webapp.
Please raise any further conversation on the onEdit() strateigiy to the Google Apps Script forum instead of here. http://www.google.com/support/forum/p/apps-script?hl=en cheers, David.
