Google is currently in the process of testing the ability to port over your mobile/home numbers into their Google voice system. It is a $20 fee payable through Google Checkout. At this current time, this function may not be available to all users as it is still in the testing phases.
All you have to do is log into your Google Voice account, check under the phone settings and see if there is a “change / port” option available (See below). Some users may or may not have this feature at this time, so if you are interested check back often.
One of the things you need to watch out for if you decide that you want to try this service is the early termination fees from your carriers. If you have a mobile phone, most likely they will hit you with early termination fees if you decide to cancel your contract.
Give Google Voice a try if you would like:
UPDATE: This may not be possible for land lines as of yet.