Today after playing around some more with the XOOM, I figured I’d try to install Google Voice one more time. Well, what do you know. It worked! By the way, this is not the modified .apk found on XDA. This is the stock version 0.3.4(#13). So if you can get your hands on a Gvoice .apk floating around, you should be good to go.
* By the way, I tried numerous times on my first Verizon XOOM with no joy. It must be the 3.1 update that’s made the difference, because this is the same .apk I used back then.