To get started, log in to your Google Voice account, go to the Settings page, and click on Change / Port next to your Google Voice number. Your mobile service plan will then be cancelled automatically (you might incur an early termination fee from your carrier).
After porting your number to Google Voice, you'll have to perform a couple more steps outlined on the Google Voice Help Center to continue making and receiving calls on your mobile device. You can begin a new contract with a different number and sign that number up under your new Google Voice number.
This feature is definitely intriguing, especially if your contract with your carrier is about to expire. For everyone else, it's something to look forward to. Unfortunately, Google Voice is still only available in the US.