This field is part of a custom list definition in a feature. Originally this was a required field. The change to not being required was done simply by removing the "Required='TRUE'" from the schema.xml, packaged as a solution, deployed via stsadm -o upgradesolution, etc.
It works as it should on my dev box but not on staging. I've compared the field element xml strings and they are identical (via SP Manager 2007).
I've tried setting it to required and back again via the UI, no change.
Any ideas?