So what changed?
1) First, the dependencies changed. In Dynamics CRM 4.0, we used forms-based authentication for IFD and in Dynamics CRM 2011 we instead take a dependency on claims-based authentication for IFD. Therefore, now it is necessary to install and configure a security token service (such as Active Directory Federation Services 2.0) and also to do more certificate management.
2) Second, configuration steps changed. In Dynamics CRM 4.0, an administrator had two options for configuring IFD. The first option was to specify the IFD settings in an XML configuration file at server installation time. The second option was to use the IFD Configuration Tool which was released out of band. In Dynamics CRM 2011, we made claims-based authentication and IFD configuration post-installation steps to obviate the need for the XML configuration file and built these wizards into our Deployment Manager tool. Administrators that would prefer to script IFD configuration can do so using the new Dynamics CRM PowerShell cmdlets.
These changes amount to a higher learning curve for configuring IFD for Dynamics CRM 2011.
I recommend that people looking to configure IFD first review the Dynamics CRM 2011 Configuring Claims-Based Authentication white paper which is posted on the same page as our Dynamics CRM 2011 Implementation Guide.