The Google Voice Blog announced that you can now use your existing phone number, with limited features, with Google Voice. Technically, I believe this is not considered porting your phone number, but it does allow you to use your existing number with Google Voice.
Google has created a nice chart showing the difference in features between using a Google Voice number or using your own number with Google Voice:
If you decide to use your existing number, make sure to read the FAQs and help documents carefully. Here is a video showing the various features you get with using your existing phone number: