To have reached this milestone means that Office 2010 is now available to a few select customers and has left the closely guarded Microsoft walls.
The CVP of Development, Microsoft Office Division had the following to say:
At this early point in our development cycle, I'm not able to share too much about Office 15, but I can tell you Office 15 is the most ambitious undertaking yet for the Office Division. With Office 15, for the first time ever, we will simultaneously update our cloud services, servers, and mobile and PC clients for Office, Office 365, Exchange, SharePoint, Lync, Project, and Visio. Quite simply, Office 15 will help people work, collaborate, and communicate smarter and faster than ever before.The public beta of Office 15 is expected to be available in late summer with the final release expected next year. While nothing much is known of the upcoming version of Office, Microsoft is planning a wide reach with a touch based version for Windows 8 and Windows Phone along with a possible release for iOS.
source - Microsoft Office Blog