I don't think the conflict is as strong as it once was. Mobile carriers have already begun to 'pivot' to be data providers instead of phone providers with data on the side. I think they'll find a way to get their margins either way and data, including voip calling, might end up being cheaper for them in the long run anyway. I guess that's contingent on if they can innovate fast enough to keep their networks from being crushed.
According to this [1], SIP support already exists in Android 2.3. I don't know how far this "support" goes (I suppose it's not hooked up to the Google Voice app, at least). And I wonder how many carriers, once they are shipping Android 2.3, are going to leave that code in place and accessible to the user.
I don't want to be too much of a conspiracy theorist here, but I do wonder if Google is dragging their feet on SIP support because of conflicts of interest with Android.
[1] http://android-developers.blogspot.com/2010/12/android-23-pl...