Om Malik: When Vonage goes portable
"[T]he company had developed a Beta version of a "softphone" a piece of software that resides on a PDA (read Pocket PC) or a Laptop and can connect to the internet using a WI-Fi connection. This also comes with its own phone number and can be mapped with your local phone number (assuming that you use Vonage phone service.) A Mac OS-X client is in the works as well and will out fairly soon."