Some Google Voice users noticed a feature that appeared in their accounts called Number Porting. This means adding your existing number to Google Voice, as opposed to having to use a new phone number entirely for the service, which users have had to do up until this point.
The feature is evidently only being tested for the time being, but will could go mainstream in the future. If you don't have to get a new number, there's a much bigger incentive to use Google Voice...that is if you don't mind paying the price.
Google requires users who have access to the feature to pay $20 via Google Checkout (another way to sneak in use of that service), then verify their phones. Google also tell users that their mobile phone service plan will be terminated when they port their number to Google Voice, which could result in an early termination fee from users' existing carriers. (see screenshot here)
Don't take this to mean that the feature makes Google your new carrier though, because you have to setup a new mobile phone service plan with your existing carrier (or a new carrier and request a new number. That number will have to be added to your Google Voice account as a forwarding phone.
Furthermore, you won't be able to receive text messages for up to three business days after the porting process is complete.
The official statement Google has been giving reporters says:
"We're continually testing new features to enhance the user experience. For a limited amount of time, we're making the Google Voice number porting process available to users. We don't have any additional details to share at this time, but plan to offer this feature to all users in the near future."