Hey all. MCD was kind enough to invite me to this thread, so here I am. I appreciate the linkage to my WPCentral articles.
It's actually a harder nut to crack than most people think. A user who only wants the game for his phone isn't going to be happy paying $15-30 for it.
Right now there isn't some magic engine that you just toss a game's code into and out pops three different versions for you to sell. In other words, it's a fair amount of work getting the same game to run on all three platforms. Windows 8 and WP8 are similar, but even then it still takes some work. I will try to work some concrete technical details into a future article (either How Microsoft can save Xbox WP Part 2 or a different one).
That said, I agree that developers should at least have the option of selling both the Windows 8 and WP8 versions together in a bundle. That's basically how Skulls' launch pricing works = $15 for both portable versions, basically the same price as the XBLA game by itself.