I have a custom object that has a lookup to Opportunity. On the field's edit page, In the new "Lookup Options" section, I can see two of the three documented options for "What to do if the lookup record is deleted:"

+ Clear the value of this field.

+ Don't allow deletion of the lookup record that's part of a lookup relationship.

However, I don't see the third option that's described in the documentation: "Delete this record also."

Per the documentation (http://developer.force.com/releases/release/Summer12/lookup+relationship+enhancements):

"Delete this record also Available only if a custom object contains the lookup relationship, not if it’s contained by a standard object. However, the lookup object can be either standard or custom. Choose when the lookup field and its associated record are tightly coupled and you want to completely delete related data."

My lookup field is defined on a custom object. Why can't i see this option when I edit the field? (I also don't see if when I create a new lookup field on the custom object.)