Microsoft Gamefest, held in London, Seattle, and Tokyo, focuses exclusively on game development for PC, Xbox, and Windows Phone. And the part we're interested in is the seminar titled 'Keep Them Coming Back: Downloadable Content on Windows Phone'.
That seminar will focus on "adding downloadable content and microtransactions to your Windows Phone games". So we can be fairly confident, given the source, that official in-app purchases will be along shortly.
Our main concern is that this official in-app solution might be limited to Xbox LIVE software. And that would be a real loss for the WP7 development community as a whole.
source: WPCentral via WMPoweruser