There's no app, yet. There's lots of debates about why - some say Google is holding off six months, till users are desperate, and to make them hate Apple. Some say the app is currently stalled in Apple's approval process. But so far, there's none.
However, you can just go to Google's mobile map site on the safari browser and then 'save it to home screen' to create a quasi-app. I've done that. Though I can't get it to recognise where I am - I need to do more fiddling with it, work it out!