Justin,
Perhaps Safari needs a developer edition
Nice idea. What I guess happens at present is that the "web kit" that drives Safari, Mail (and iTunes?) is the important part of the update, and this is something that is available system-wide. So there would have to be a way of running different web kits concurrently. (Might even be able to do it from the same UI, as you suggest.)
-Hugh Todd
*****************************************************
The discussion list for http://webstandardsgroup.org/
*****************************************************