I've read that when I customize the quick access toolbar that this is saved
in a .qat file. Where is this file?

I've looked in the following location both for "All Users" and specific to
my user id:
C:\Documents and Settings\All Users\Application Data\Microsoft

In addition, I've done a search on *.qat with no result

Once I get the .qat file, can I save it for other apps or do I need to
customize each app (since some commands aren't in all apps i.e. functions)?