A useful collection of info regarding co-authoring in SP 2010: "In today’s highly connected work environment, documents created by multiple authors, editors, and stakeholders are becoming the rule, instead of the exception. Organizations look to the communication and collaboration capabilities of Microsoft SharePoint Server 2010 to help them foster communication and collaboration between end-users while reducing administration required to support it. Microsoft Office 2010 continues this trend with co-authoring functionality for Microsoft PowerPoint 2010, Microsoft Word 2010, and Microsoft OneNote 2010 documents on SharePoint Server 2010. Co-authoring removes barriers to server-based document collaboration and helps organizations to reduce the overhead associated with traditional document sharing through attachments. Co-authoring simplifies collaboration by enabling multiple users to work productively on the same document without intruding on one another’s work or locking one another out. This functionality requires no additional server setup and is the default state for documents stored in SharePoint Server 2010. Co-authoring functionality is managed by using the same tools and technologies that are already used to manage SharePoint, helping to minimize the impact on administrators."
Co-authoring overview (SharePoint Server 2010) http://technet.microsoft.com/en-us/library/ff718249.aspx
Co-authoring administration (SharePoint Server 2010) http://technet.microsoft.com/en-us/library/ff718235.aspx
Configure versioning for co-authoring (SharePoint Server 2010) http://technet.microsoft.com/en-us/library/ff718246.aspx
Configure the co-authoring versioning period (SharePoint Server 2010) http://technet.microsoft.com/en-us/library/ff718243.aspx
Configure the maximum number of co-authoring authors (SharePoint Server 2010) http://technet.microsoft.com/en-us/library/ff718248.aspx
Disable co-authoring (SharePoint Server 2010) http://technet.microsoft.com/en-us/library/ff718245.aspx