Yesterday at the PDC2008 conference, we announced Office 14 for the Web. There is a great Channel 9 video with a demo:
There are also some good screen shots on Joel Oleson's blog:
Office 14 will support rich desktop client, Web browser, and mobile versions of Word, PowerPoint, Excel, and OneNote. The browser versions will be cross platform and cross browser. These versions will also support simultaneous editors of a single document across the three client choices.
The Web browser versions will be available through OfficeLive in an ad supported model, as an online subscription or via SharePoint 14.