Google Maps is finally rendering their maps in retina mode on my rMBP. I noticed it yesterday... the maps look really detailed and crisp, along the lines of what the maps app looks like on the iPad. I didn't see any threads about this, and I searched, so I apologize if this is old news. I've only used Safari on the Maps, so I can't confirm if they render in retina in Chrome/Firefox.
http://maps.google.com
http://maps.google.com