Unlike in SQL 2005, when developing SSIS packages in SQL Server 2008 BIDS you must have the SSIS service installed on your workstation. This is a change from SQL Server 2005 where you did not have to have the actual SSIS service installed on your workstation.
It doesn’t appear that you actually need to have the service running. I have stopped the SSIS service on my workstation and I am still able to run my SQL 2008 SSIS packages, however I would recommend setting it to manual rather then disabled just in case BIDS needs it running for some reason.
If you try and edit an existing package without the SSIS service installed an error message will be displayed saying that the service needs to be installed.