This is a very cool new feature for Google Maps. Although still only in the US and Japan right now, I hope to see it become available in Australia soon. The new feature is starting to show interior maps of airports, shopping centres and retail stores in the US and Japan. In Japan transit stations are also mapped. Find out more in this video below. Businesses in the US can help growth of this by uploading their floor plans.
It needs to be noted though that Bing Maps came out with 'Mall Maps' in December 2010.