Trusted XP Install Scripts

[colinp]

I need to know if there is an "always trust xpi's from" feature? 
Basically we want to have a preference (in prefs.js) or somewhere that 
allows us to run xpi's without ever prompting the user.  This way if the 
content comes from a particular source, we can assume that it's trusted 
and just simply install it, rather than prompting the user.

This would not require signed scripts, ideally it would only require a 
simple preference in the prefs.js file.