Location of Maintenance Plan's Back Up Database Tasks information (SQL Server 2005)
In SQL 2000 they were in the msdb database.

Specific table information can be found here:
http://www.toadworld.com/platforms/sql-server/w/wiki/9438.msdb-databases.aspx#Database_Maintenance_Plan_Tables

In SQL 2005 maintenance plans are modified SSIS (or DTS) packages and are stored in the dbo.sysdtspackages90 table.  (from: http://www.sqlservercentral.com/Forums/Topic501078-5-1.aspx )

In SQL 2008 and 2012 they are in the sysssispackages table.