The good news for Windows Phone developers is that support for in-app purchases is being added to your platform of choice, helping to level the playing field with Android and iOS. Unfortunately, the same courtesy isn't being extended to Windows Phone 7.8, a move that will leave existing WP users and developers in the cold, provided they're interested in the whole upsell business to begin with.
Microsoft confirmed the in-app roll out on its Windows Phone Dev Center page, noting that "you can only add in-app products to Windows Phone 8 apps." Developers who choose to implement in-app purchases to their apps are asked to provide the "fundamentals of your in-app product -- alias, price, type, and language(s)."
Why not invite Windows Phone 7.8 users and developers to the in-app purchase party? The Verge points out Microsoft is introducing its Wallet Hub feature in Windows Phone 8, which will be the one and only way Redmond supports in-app purchases, among other features.