Google Voice (voicemail) now works with existing phone numbers
But there's one relatively high barrier to entry. Well, two if you count the fact that it's still in private beta. But the main barrier is that while people can ring your existing number by dialing a Google Voice number, up until now it's been a one way street. They couldn't dial your cellphone number and automatically transfer into your Google Voice account. And that's meant that you probably had to maintain two or more voicemail accounts.
But now Google is taking a baby step toward number portability. When new users sign up, they have a choice of asking for a new Google Voice number or using an existing phone number.
While you won't get all of the features of Google Voice if you sign up with an existing number, you will be able to direct all of your voicemail messages to your Google Voice inbox. What this version of Google Voice doesn't offer is the ability to have all calls made to that number forwarded to multiple phones.
If you already have a Google Voice account and phone number, you can add Google voicemail to existing mobile phones from supported carriers. This doesn't appear to be an option for non-mobile phones. For instance, I have Google Voice configured to ring my SkypeIn number and I would love to consolidate my Skype and Google voicemail boxes, but there's no easy way to do that yet.