Regarding desktop files in /usr/share/applications:
In F14 if you navigated to this directory with nautilus and right clicked on a file you were given the opportunity to edit it with gedit.
I find that in F16 nautilus merely gives you the opportunity to "open" the file. If you do, instead of opening gedit on the file you are presented with something like the "about" message typically displayed with an application.
But if you go - command line - to the directory and type
you get to edit the file as before.