A Windows Phone principal program manager has left the company two weeks after making headlines for tweeting his impressions of an unreleased Nokia phone running the Microsoft mobile operating system.

A Microsoft spokeswoman confirmed this morning that Joe Marini, principal program manager for the Windows Phone web platform, no longer works for the company, but declined to provide further details, citing a policy against discussing personnel matters.

But the back story, as we understand it from people inside the company, is that Marini resigned after learning that he would be let go for improper use of social media and disclosure of confidential information.

We’ve reached out to Marini and will update this post when we hear back.

The tweets are still online, starting with this one: “I just got a chance to try out one of the slickest looking # Nokia phones I have ever seen. Soon, you will too! # wp7”

Asked to rate it on a scale of 1-10, he replied, “Overall I would say an 8. Solid feel, good camera, responsive UI, and nice little touches on the body construction.”

Explaining why he didn’t give it a higher score, and whether the camera was as good as people expected, he tweeted, “Yeah, the camera was good, but I didn’t have optimal lighting. I’d like a larger screen too.”

Microsoft’s social media and blogging policy (PDF) calls on its employees to “be smart” and not disclose information in tweets or posts that would otherwise be considered confidential.

Marini had been at Microsoft since 2003, and has given a variety of public talks over the years, including one at Microsoft’s Build conference in Anaheim last week.

The company has been taking harder line on leaks in general recently, including dismissing one journalist from the Build conference for prematurely posting video content about the Windows 8 operating system after agreeing to an embargo.