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
In Object Explorer, right-click a server and select Properties.
Click the Database settings node.
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.