I was unable to answer this question at my Providence, RI event this week regarding changing the backup dataretention period.  Here's the answer I got from where else but the SQL2005 Online Books:

To set the backup retention duration

  1. In Object Explorer, right-click a server and select Properties.

  2. Click the Database settings node.

  3. Under Backup/Restore, in the Default backup media retention box, type or select a value from 0 through 365 to set the number of days the backup medium will be retained after a database or transaction log backup.

    The default value is 0 days.