For current Google Voice users, this means you'll no longer have to remember and use two separate numbers, one direct line and one Google Voice line, to get all of Google Voice's features, including visual voicemail, conference calling, call screening and more.
Last year, when Google Voice was still invite-only, Google announced some similar features that allowed users to use their own phone numbers with Google Voice, but it wasn't a full number porting and didn't bring the full depth and breadth of Google Voice features.
Actual number porting was rumored to be "coming soon" last week; and as of today, it's live and available for all current Google Voice users. The feature will be available for new Google Voice users in the coming weeks.
The process of porting is a bit of a rigamarole at the moment. Your mobile provider has to cancel and restart your service, and during the porting process (which can take up to three days), you might miss a few SMS or MMS messages.
Here's a brief video from Google showing roughly how the process will work.