Despite loading the Google Maps API, via this url:
http://maps.gstatic.com/intl/en_ALL/mapfiles/276b/maps2.api/main.js
I'm getting the above error.
Why is it that when I download that URL in my browser and do a find for 'GOverlay' I'm getting zero matches?
Have Google removed this from their API or something, causing all my code to break?
GOverlay is an essential part of the Google Maps API implementation, see the V2 documentation for GOverlay here.
The reason for you not finding "GOverlay" when searching through the Javascript file you provided is simply that the Google Maps API consists of several Javascript files, not all of the code is in main.js. Additionally the code is obfuscated which could mean the build GOverlay by concatenating some crazy strings.
On a basic note: Why do you want to use some static JS file? The offical way to use the Google Maps API is using a key, which you have to obtain by registering with your Google account. So actually the URL you should be using is:
http://maps.google.com/maps?file=api&v=2&key=abcdefg
Or use the AJAX loader as seen on the Google Maps V2 documentation here.
The code is actually loaded asynchronously.
Post a link to your page - it is more than likely that there's some issue with your code.