Microsoft has had co-authoring available in Office since Office 2010, but not in Office 365 or the Office Web Apps. Over the last few years the Office Web Apps have gained more features and have become increasingly similar to the desktop applications. With the introduction of Office 365 the pace of features introduced has accelerated and real time co-authoring is finally coming to the office web apps. The following video provides a demo of how the co-authoring will work, starts around 5:35
The next version of Office is code-named Gemini and is expected to introduce Metro Apps for all applications in Office. It was also recently announced that Outlook RT will come with Windows 8.1.