I wrote a little about this on one of the other (non-Android) MR places, but I thought I should also post this here.
Try Talkatone. If you have Google Voice with a US phone number, and have set GV up to forward to Google Chat, you can both make and receive calls on your N7.
I live in Canada, and people have to call a US number to get me this way. Not a problem for my family, who are all in the US, but this could be a deal breaker for other Canadians. Of course, I still have a cell phone and a landline, so I suppose they can continue to call my Canadian number using one of those
The big deal is that I can call anyone in the US or Canada over WiFi using GV on my cell phone and now, on the N7.