IBM has released Cognos BI 8.4.1 and they claim that it finally supports SSAS 2008. I will be upgrading a client in early January 2010 and put this to the test. Prior to 8.4.1 there was a way to access an SSAS 2008 cube but it would not pass the user credentials with Kerberos as it does with SSAS 2005. I will be testing the new release for integrated security in particular.
I just learned that the support for SSAS 2008 will require a fix pack in addition to the 8.4.1 upgrade.
After upgrading and applying the patch, SQL Server Analysis Services 2008 is now in the drop down for data sources. I was able to successfully create a data source set to use the external namespace of Active Directory and browse the cube passing my credentials.