I see that Google China has added maps to their list of services:
Most cities don't have much detail, but major roads appear to be in place. Cities like Shanghai and Beijing have a bit more mapped out.
The service seems popular as everything is very slow to access, at least for me from Japan. I am seeing them making access calls to this address: mapgoogle.mapabc.com
So Google is using a 3rd party for their Chinese language map overlays?