The first of the universal apps started showing up in the Windows Store earlier this week. The first two paid apps to take advantage are 17-Bit's Skulls of the Shogun and Microsoft's Halo: Spartan Assault. Each game costs $4.99 and can be purchased through either the Windows Store or Windows Phone Store.The Xbox version is still a separate purchase, though.
In addition to the paid games, Microsoft has also turned several of its other games into universal apps, including Wordament, Hexic, Mahjong, Minesweeper and Solitaire. Given that the games were already free the move is largely symbolic, but it is still a good first step.
By giving developers the option of universal apps, Microsoft is starting to do for apps what digital publishing is still trying to pull off with ebooks. It's been 6 years since Epub was first demoed on the Sony Reader, and there is still no true universal ebook format.
In addition to Amazon's own unique Kindle format, there are at least 2 additional camps in the ebook market: Apple and Adept (Adobe DRM). They are as divided by their DRM as they are different from the Kindle format, and we are all the worse for it.