This has been raised since SQL Server 2005 and appears it is still a bug in SQL Server 2008 R2. Other reports are 295846 and 454073.
When changing the owner of a SQL job to "sa" for example and then later save the maintenance plan that the job is part of the owner of the SQL job reverts back to the user that saved the maintenance plan, in my case my domain account.
This is not ideal in that when users leave the company the jobs fail. I have seen there are some work arounds to this but why is this not being fixed in any service packs and or upgrades?