Read the title carefully and take it literally. I had discussions with people trying to convince me this is indeed possible. What they were trying to explain to me is the following:
Let's take a SQL Server Management Studio 2008 and connect to a SQL Server Integration Services 2005 instance. Proof can be found here.
It won't work. One would think it's backward compatible but it's not.
However, there is somewhat a workaround: connect to SSIS 2008 instance through SSMS 2008 which of course should work AND within the stored packages node, you are indeed able to connect to 2005 packages.
Literally, SSMS 2008 cannot connect to a SSIS 2005 instance