For those who are struggling with the BCS framework read and write to SQL Server or LOB data source concept using tools such as SharePoint Designer 2010. This video I found is really helpful in providing a comprehensive understanding of the different ways in which SharePoint Server 2010 Business Connectivity Services (BCS) authenticates a users identity and explains the “double-hop” scenario and ways to overcome this.

http://office.microsoft.com/en-au/sharepoint-designer-help/video-business-connectivity-services-authentication-from-the-browser-VA101873629.aspx?pid=CH010373542&CTT=4