Slap some fixed positioning on the Google Maps iframe, some relative positioning on your content container, add a bit of z-index trickery and some CSS on the body and html tags and you're done. Instant Google Maps as a background. Fully interactive, of course!
Just don't turn on Google Earth view if you have any rounded corners. That is unless you like having chopped off edges. In which case, go ahead, knock yourselves out!
The next demo will show how using the Google Maps API you can edit the look of the background map to match the rest of the web page.