Sprint is the one carrier so far to offer deeper Google Voice integration, making it easier to sign-up/port your number (and cheaper, as porting through Sprint won't cost you the regular $20 fee), and offering MMS-to-e-mail forwarding to effectively create an MMS system. Now, the Group Product Manager for Google Voice, Vincent Pacquet, has said that the company is "having discussions with other carriers about this".
This report falls under the same heading as the report saying Google is planning to offer revenue sharing with Google Wallet as an incentive to get on more carriers' devices. Of course, Google will offer revenue sharing, and of course Google will "talk" to carriers about getting more Google Voice integration, but there's no benefit for the carriers. Verizon, AT&T, and T-Mobile are all investing in ISIS, so won't be too willing to support Wallet, and similarly, those carriers make a lot of money on SMS/MMS add-ons, so why would they support Google Voice and hurt their own revenues? Sprint was willing to do it, because the company lives and dies on its "unlimited everything" plan, but until more customers are opting for the unlimited everything plans on other carriers, we wouldn't expect too much headway made on this for Google.