In my previous post, I included a screenshot of a prototype, but glossed over what it actually does. Given an OCLC record number and a ZIP code, it plots the locations of the nearest holdings of that item on a Google Map. Pulled off in Python (as all good mashups should be), along with SIMILE Exhibit, it uses the following modules:
geopy simplejson web.py and, of course, worldcat. If you want to try it out, head on over here. The curent of the code will soon be able as part of the examples directory in the distribution for worldcat, which can be found in my Subversion repository.