-- For 2005, here is a simple script:
use msdb;
go
updatedbo.sysdtspackages90
setownersid = (
select sid from msdb.sys.syslogins where name = 'LoginNameHere')
where [name] = 'MaintenancePlanNameHere';
--------------------------------------
-- and for 2000,
use msdb;
go
update sysdbmaintplans
set owner = 'LoginNameHere'
where plan_name = 'MaintenancePlanNameHere';