Now, there's already a Google Calendar for the iPhone, and a way to update your Google Calendar via SMS, but we're talking about real push synchronization here.
That means your Gmail address book and your Google Calendar will be synchronized automatically with your phone, over the air, all the time. Change an event on your phone and it'll be updated on the web, and vice versa.
Depending on what type of user you are, this is a major improvement. For business users, it's vital. For the rest of us, it may not be as important, but it's one care less in the world.
Google Sync is already available for the BlackBerry and Symbian S60; if you have a device that supports the open SyncML protocol, Google Sync will also work, albeit only for contacts and not for the calendar.
See how Google Sync works in an introductory video below: