I've got to admit, I'm pretty excited about SimCity - every trailer so far has been better and better, and this trailer is no different. This one goes into some more detail of how surrounding city interactions work, and more.
As you can see in the video above, you can work with neighbor cities, or friends cities. The video has a great example of using a Fire Department from another city - in the case that you didn't have one, a neighboring city will come over and put out your fires for you, all without any user interaction.
I'm loving the upgrades to buildings, and the very social aspects of the game. I'm incredibly excited to see what the final game turns out to be!