Waze and Inrix touted a similar feature last September, so Google may be trying to ensure that it at least keeps up. Waze's app can actually find you a spot, however, while Google's app just gives a general overview of the situation. It's not clear yet how Google is getting the data.

I downloaded the app and parking availability didn't work anywhere I tried in Paris, France, and Ars Technica says it's not appearing in the New York/Long Island area either. However, it did show up for me at the destinations that Android Police's tipster tried (somewhere in Maryland, I think). So far, locations seem to be limited to malls, airports and other public spots, which is not surprising considering that the feature is still in beta. It may work for you, though -- you can download it from the APK Mirror if you're feeling adventurous, or wait for the official release.