We'll be extending an Ext.Panel object from Sencha Touch to create an iPad signature pad. When you click submit it will tie in to the server-side PHP script in this post. It works pretty well, though there are many ways that it could be enhanced.
We are using the Sencha Touch MVC style approach outlined here:
Go ahead and try it out here (iPad/Safari/Chrome/Webkit only): http://www.jamund.com/signature