I use GrooveIP ($5 on Android Market) to use Google Voice over the data connection on both my actual Android phone and my ASUS Transformer tablet/netbook (which has no cellphone chip at all inside of it, just wifi).
Granted, it would be nice if this were more of an integrated part of the OS and the Voice app, without needing to set up or use a third party SIP provider and it all Just Worked including seemlessly handling incoming calls, but there are some options out there now to get this sort of setup working.
Granted, it would be nice if this were more of an integrated part of the OS and the Voice app, without needing to set up or use a third party SIP provider and it all Just Worked including seemlessly handling incoming calls, but there are some options out there now to get this sort of setup working.