For years, Nokia users were downloading applications from the Ovi Store. A few days ago, Nokia CEO announced that Nokia and Windows will bring a Nokia-branded application store. That store will feature both. applications compatible with Nokia and Windows devices. This suggests that this store shall arrive at the announcement of a Windows Phone.
The operator billing will still be from Nokia for the user convenience. The first Windows Phone is scheduled for the end of 2011 is you listen to rumours. Other rumours from other mobile websites claim that the devices will first make appearance in the 2012 MWC.