Thought that this was a Major feature for the Office 2010 Web Apps. But it did not make it in the final bits for PowerPoint and Word.
“Simultaneous editing for collaboration is one of the most hyped features in Office 2010, however it’s only supported over the web in the Excel 2010 web app. For Word and PowerPoint simultaneous editing, you’ll need to have the full client versions of the Office 2010 products. This will likely be a disappointment for people who were hoping to collaborate on documents from kiosks anywhere in the world.” (Michael Fettner)
Go over to his blog for the details.