It's impossible as is.

When you request the script from Google, they send headers along the script and those headers contains the no-cache directive.

So if you want them to be cachable, you must create a proxy. Instead of pointing the script src at Google, you point it to your server. Your server then make the call to Google and send the response back to the client.

This way you'll have control over the HTTP header and the caching. You could do caching on the script content as well to make less connections to Google.

I wouldn't advise someone to do it on a production or a mission critical website. All Google APIs are updated frequently and are bound more or less together. If something goes out of synch with something else, you have a hard to track bug on your hands.

Hope that helps.

EDIT: I heard you were putting your scripts in the HEAD section of your document. Maybe thats hurting your perceived page loading time. Try to move the download of the script just before the </body> tag and the map initialization in the onload event of the page.

Mike