To many, this is a non-issue, since SSIS is installed anyhow. But not everyody installs SSIS. I for instance prefer to keep my production systems as clean as possible and only have what I really need (a principle which is harder and harder to live after as years go by...). Also, not all Editions of SQL Server comes with SSIS.
I did a test some months ago for SQL Server 2005 with a recent sp and also on SQL Server 2008. SQL Server 2008 did require SSIS (I tested both without and with SSIS installed), where 2005 sp2 didn't. I recently learned from Phil Brammer in MVP group that there has been progress. Here's the run-down, you don't need SSIS installed to execute maint plans:
SQL Server 2005 requires sp2.
SQL Server 2008 requires CU3 or sp1 (see http://support.microsoft.com/kb/961126/). I dodn't test this combo (2008 sp1 without SSIS), but I have no reason to doubt that KB article.