To be honest...I don't see why MS would have an issue with allowing Sony to push its WP handsets with PSN instead of XBL. First they let it happen with China-bound handsets (replacing XBL with some local network). Also, it doesn't seem as if the XBOX department is obliged to worry about the Windows Phone division. There are XBL titles on iOS, including the companion app no less. Why should the WP division even care about these other divisions when they don't fully care about WP?
If Sony wants to push a Windows Phone geared towards strengthening PlayStation in the living room, then go right ahead. Based on the number of friends I have with XBOX, I don't see the XBL crowd shrinking in the future anyways, and I doubt they'll buy a Sony WP if they wanted an XBOX WP device. If you want the XBOX crowd, then you need to make a direct XBOX branded phone - and above all, you need games on par/superior to Infinity Blade available on such devices.
Let Sony integrate its services deep into the WP system, e.g. a Sony Store section in the Marketplace, Qriocity app, PSN integration, etc. If they want to equip Windows 8 tablets with such features as well, then let them do it. At least you can entice the PlayStation crowd into Windows Phone/tablet family, that might help strengthen the front against Apple when they invade the Living Room. But as a matter of principle, do what you need to do to make Windows Phone succeed, it is NOT stopping XBL from doing the same with iOS, Lync on iOS, Photosynth and SkyDrive, etc.