Windows Phone 7 has been released to manufacturers, Microsoft has announced in an official blog.

The milestone means manufacturers can integrate the software with their hardware and networks to create smartphones and devices for customers prior to the holiday selling season, a Microsoft spokeswoman said.

In the blog, Terry Myerson, corporate vice president of Windows Phone Engineering, said 10,000 prototype devices ran automated tests of the OS daily, with over 3.5 million hours of stress tests. "We are ready," he said.

Since Microsoft reached the Technical Preview stage in July for the OS, engineers got feedback and changed the Facebook integration so users can filter contacts so the friends they really know show up in the list. Microsoft also made it easy to "like" a post from the People Hub in the OS, or quickly post a message to someone's Facebook wall directly.

Microsoft as of late July described at least five makers for Windows Phone 7: Asus, HTC, LG, Samsung and Dell .

But a Microsoft spokeswoman today reiterated that the company is only officially naming partners touted at Mobile World Congress in February. That list adds Sony Ericsson and Toshiba to the five named by a Microsoft official last month.