It enables push notifications for contacts and calendars (on both the iPhone and Windows Mobile)!
I’ve been waiting for this one for quite a while. Currently I used a complex scheme of Outlook, Google Calendar Sync, and still don’t get all the great contact functionality (I could, but it was too annoying to setup — plus I hate all of the useless Gmail contacts).
If you have push notification enabled on your iPhone, you get push updates.
This is essentially an extension of the push functionality for Exchange servers.
Some others have posted on this:
In looking at the instructions, they say that all of your contacts and calendar information will be deleted. You should backup your data:
Below is a video of how Google Sync works: