Using API from code injected by Grease Monkey

I'm trying to inject maps on a site using Grease Monkey with Firefox
1.5. This is an entirely client-side endeavor, pulling together real
estate data, geocoding calls, and (hopefully) Google Maps onto the
pages of the real estate site. I've see the thread "ANN: Adding maps
to real estate application", so there may have been a way to do this
with one of the previous versions of GMaps (or Grease Monkey, or
Firefox). In any event, both the code I found there and my own code
lead to the same error on attempting to load the API, which is:

_mNavteq is undefined
http://maps.google.com/mapfiles/maps.28.js

at line 561. Even document.write()-ing the sample code from the API
documentation into an IFrame didn't change the situation. In addition,
attempting to load the maps API seems to trigger a reload of the
top-level page itself. Perhaps this is involved in verifying the
domain with the key?

Is there a way to use the Google Maps API from injected Grease Monkey
JavaScript?