From Arkadium's press release:
"We are very excited to be working with Microsoft Studios to develop casual and social cross-platform experiences," said Kenny Rosenblatt, CEO of Arkadium. "We have been looking to strategically partner with a company that can help catapult our business and accelerate Arkadium’s cross platform game development. Microsoft is the perfect partner to help us realize our growth strategy."
Having worked with Microsoft for a number of years, this is a smart move from Redmond. Games are going to be a big part of the ecosystem as Windows Phone expands, so to have solid partners on board with the experience of Arkadium (they're behind Writers Blox on Facebook and Mahjongg Dimensions on iOS) is a smart move.
Hat tip to WMPowerUser.