Different carriers have different support for Google Voice. Sprint has native Google Voice integration where your GV number is your carrier number.
On other carriers the integration isn't as tight, but the experience is still superior to the iPhone Google Voice experience in that Google Voice on Android integrates with the standard phone dialer so you just make outgoing calls like you normally would using the standard dialer (or any other app which invokes the standard dialer) and they go through Google Voice.
