Called Comcast Customer Service today at 1-800-934-6489. Follow prompts for help about Internet to get to the right spot. Eventually spoke with a rep named Mohamed at the Schaumburg IL call center. He said Comcast is actively working on an Xfinity TV App for Windows Phone right now, and that Comcast currently expects an "early 2013" release. No idea what Windows Phone OS version numbers would be supported (will the WP early adopters still on contract get hosed again?). No idea on feature support (e.g. DVR Management, Streampix...etc.) The Comcast representative opened a ticket for the Xfinity Windows Phone App issue, without any trouble at all.
By the way, there is a partial Comcast Xfinity TV App workaround for the unloved Windows Phone user base. You can use the web. Use IE on your phone. Navigate to www.comcast.net At the very top of the landing web page there is a pretty dark and hard to see menu. Select the TV button on that menu. Then, select the DVR Manager tab. Login with your Comcast account. The website wasn't the greatest experience I ever had, but at least it lets you accomplish something like managing a DVR. The DVR management feature is especially important because Comcast has this awful limitation (as of 12/2012) on the whole house DVR system I got from them. You can only delete a recorded show from the DVR itself --- meaning you need to be in front of the DVR. If you are in front of a Set Top Box somewhere else in your home (not the master DVR), you can't delete a recorded show. The iPhone app and the website will let you delete. Crazy! I chatted with Comcast about this issue, and they evidently know about it, but do not have a release date on a solution.
Hope this helps. Good thread OP. Post your experiences so we can know if or when the app is coming.