Google Voice recently started offering number porting, but in a limited test. Today, number porting is available for all users. Current Voice users can port their cell phone number to Google Voice for a $US20 fee, so none of your contacts have to learn a new number for you.
We explained the ins and outs of the process here. Note that the feature is only available to current Voice users; new users should get the feature in a couple days. [Google Voice Blog]