Google just announced it's own OpenID initiative - they've joined the board of the OpenID Foundation and have rolled out a Google Federated Services API.  Along with that, they've also anounced a beta of the OpenID Provider for Google accounts, found here.

I played with it some this evening and I wasn't able to use the Google provided URL to login.  Now, that could be because I'm only testing from localhost and not an internet page.  I got, "There was an error in authenticating you" when I wasn't getting 500 errors from Google.  Your mileage will probably vary.

With that said, I don't think it would be terribly difficult to write a Google Account Provider, but I wouldn't expect it in the core (you could sponsor somebody to write this for the core...).  If nothing else, watch snowcovered or the marketplace for one - it'll probably appear for $19.99 on one or the other.

I've not looked at the source of any of the providers, but there's no reason why you couldn't pick up the return URL.  But I would suspect that there's a problem with implementing it this way:  I use Live Authentication on a number of DNN sites and all of them redirect you back to the main page rather than to the return URL.  On at least one of those sites, using ASP.Net authentication redirects you to the return URL.