As you already know, one of the new feature provided by SharePoint 2010 is the Office Web Applications (additional product to be installed) to allow users to display and edit Office document with a browser.

However, if you configure manually – I mean without running the SharePoint wizard – the Office Web Applications you may have trouble when you try to open Office document in the browser.

Indeed, if you take a look at the Application event log on the SharePoint Server, you will see an error message which says the application pool user account need additional rights on the content database.

You have to open the SQL Management Studio console, locate ALL you SharePoint content databases (included the one used by the SharePoint Central Administration) and add the dbo right for all service account used by the Office Web Application.