See screenshot. In the "Set all values to" row, there's that unlabeled checkbox. I had no clue what that was, and was wondering where the "Unset All" control is. Well turns out that's what the checkbox is for, but it needs to be properly labeled. I suggest "Unset All" for the label, on the left side of the checkbox.
Done - r3399.
Fixed, rev3416
This bug was previously known as http://jira.rhq-project.org/browse/RHQ-1755 Imported an attachment (id=368635)