Google's details on maps free usage limits are here and seem to imply the 25,000 map loads per day are per API key:
However, if you place a Google Map on a Visualforce page in Salesforce, even the first load of that page will give you the following error:
The URL of my page is on a Salesforce Developer Edition org:
Why am I immediately over quota? Is it because map loads are counted not just per API key but also per IP address of the server or per second level domain name (eg force.com) and all my neighbours on Salesforce.com have already shot through 25K map loads for the day before I run my page?