Office 365 - Office Web Apps: More Office, more collaborative, more devices

Check the link below for more details (e.g., editing from Android tablets) and a video of co-authoring in PowerPoint Web App; BTW you can freely use Office Web Apps in conjunction with a SkyDrive account

"Our goal is to deliver Office Web Apps that people can rely on to create polished Office documents from start to finish, all from the Web. Since we first launched Office Web Apps, browser technologies and speeds have advanced and the development infrastructure has matured, making the Web a better platform on which to build Office and more quickly innovate on the Web. With these improvements, we're better able to deliver a comprehensive productivity experience with the Office Web Apps.
To realize our goal of a great Office experience on the Web we’re investing in three areas: user experience, social and collaboration, and broad cross platform browser support. You’ll see us introducing a collection of authoring features that make it easier to collaborate in the cloud, a new real time co-authoring experience and editing capabilities from more devices."

Office 365 - Office Web Apps: More Office, more collaborative, more devices