If you use their web application to place calls and send SMS, then yes it's "VoIP" in the sense that it's using the data connection.
The native applications for various platforms, however, seem to involve actually using the phone as a phone, but with the calls routed through GV in order to have the caller ID and such work as expected. IIRC some earlier efforts involved explicitly dialling your own GV number to initiate the call.
VoIP means voice over IP. The only time your voice goes over IP with Google Voice is on their internal network. You use a regular phone to call Google Voice, and then they make a regular call to your destination.
So if people get used to using GV on iPhone, Blackberry and Palm, it would make switching to GV on Android trivial. There would almost no transition costs.
I get tired of my iPhone (or more likely pissed off at Apple for something) then I could just take my GV #, make some settings changes, then buy an Android phone and be off.
Easier switching is better for us, maybe not so much for Apple or AT&T.
The native applications for various platforms, however, seem to involve actually using the phone as a phone, but with the calls routed through GV in order to have the caller ID and such work as expected. IIRC some earlier efforts involved explicitly dialling your own GV number to initiate the call.