The update is version 3.2 and includes one major new feature, layers. You can add layers of data on top of the map. The easiest way to explain it, is with this video:
The second change is a fix for the internal GPS/Google Maps. Tom explained there was an issue "for mobile issues certain BlackBerry devices were experiencing, namely the Tour, 8330m, and others on certain carriers."
Forum discussion at Google Mobile Help.