Google Voice gives you a new phone number to hand out. Many people have wanted instead to have Google Voice features on their current phone number that their friends and customers already know.
Google has said that capability is coming. It will take, I think, negotiation with the telcoms, and I'm not sure the telcoms will want to give up the customer control inherent in controlling the phone numbers. Google may surprise me, but I don't expect to see comprehensive number porting anytime soon.
Today, Google announced an almost number porting ability. In a nutshell, they tell you how to auto-forward your cell phone calls that would normally go to your cell's voicemail, to Google Voice.
Their help system shows that a lot of advantages of Google Voice are lost when you do this. If you already have a Google Voice number, you can choose to have your cell phone routed to Google Voice. When I tried to do this, they warned me Sprint might charge me for each voice mail until Sprint finishes rolling out the call forwarding abilities.
I think for now, I won't make use of this. Having a unique number I can use is my preferable way of using Google Voice anyway.
Update 10-28-2009: Tech Crunch reports Sprint does charge 20 cents a minute for forwarded calls, but are going to make it free in mid-November. Then I can use this feature.
Update 10-31-2009: New York Time is calling this Google Voice Lite. You get to use your current cell phone number with a sub-set of Google Voice features.