Maybe some of the new developer tools help here? For instance, in the web console the network links are clickable and will show request and response headers. Maybe that could be extended to show the data transferred as well.
if you check "Log Request and Response Bodies" (Web Console's Context Menu) you can view the details of a request in a network inspector panel.
There are likely to be all sorts of performance problems with storing the original source just in case someone later wants to view what it was to start with. Also there are probably cases where what we really need is the state at onload or some other milestone rather than what goes across the wire. Also the 'original' state can be discovered using some form of network proxy without too much difficulty. Hence I think there is probably a better way to solve this - how about using Firebug's ability to break on DOM events to watch what's going on?
original test case links are gone. I'm marking this WONTFIX.