Don't even TRY it when both people have Google Voice. Well, do try it, and cry when it says FaceTime failed.
Reason why this is interesting: Google Voice routes your calls through GV's servers to your phone, so it's more-or-less a standard phone call. GV is also a shield so you don't have to give out your "real" phone number to people. With FaceTime not supporting GV, you're kinda unprotected. Here's the error when you try to FaceTime a Google Voice number from a regular number, also when you try to initiate a FaceTime when you're already in a call with a Google Voice number.
Also interesting, Gartenberg says that FaceTime does work over GV for him. Can anyone verify this on your own phones? It does not work for us.
Update: It seems like some commenters are confused about what Google Voice is for. It's not a VoIP service, it's not another communication service. All it does is route calls, in essence, from a Google Voice number to your phone. Just one more layer inbetween a call. Not a VoIP chat, not anything complicated. That is why there's the expectation that this will work.
Update 2: A-ha! Just found out how it works, thanks to a tip by reader Brian. Here's what you do.
- Have the person without Google Voice start a normal call the person with Google Voice. If you both have Google Voice, and have the numbers programmed in, this will not work at all
- Once the normal cell call is established, have guy with Google Voice press the FaceTime button
- The guy without FaceTime (the initiator of the call) will see a FaceTime request come in. However, this will be from the Google Voice guy's real number, not the Google Voice number
So there are some drawbacks to this. It can only be done if a guy who doesn't have Google Voice initiates the call. If the Google Voice guy starts the call with his GV number, it will fail. Also, this exposes the real number to the recipient, bypassing your Google Voice number entirely. This is how it actually works, but kills the point of Google Voice. However, it's better than not having FaceTime, so at least it kinda works, if you go about it a certain way—which essentially is not using Google Voice at all. So our original point still stands: FaceTime doesn't work with Google Voice.
Also, you can't both have Google Voice. If you do, the GV system will think you're making a call from the Google Voice number even if you're calling from your regular number (this is a feature). And the FaceTime request will get lost in the ether, since it's sent to your GV number and not your real number.