The news is hot on the heels of Nokia's announcement to switch to Windows Phone as primary smartphone platform. In addition to Nokia, and possibly Acer, Windows Phone has the following OEM partners: HTC, Dell, Samsung, and LG.
Microsoft had four OEMs onboard in 2010 and could have six in 2011, as well as a significantly larger opportunity to grow its market share in the mobile space. The only major OEM that is missing from the list is Sony Ericsson.
That company seems to be strongly behind Android and doesn't seem interested in Windows Phone whatsoever. This is not too surprising, given that Sony and Microsoft compete in many different areas. With Sony Ericsson launching the Xperia Play, the first PlayStation Certified device, it would be odd for the company to also push out a WP7 device, which includes Xbox Live.