Microsoft CRM 4.0 form attribute mysteriously read-only

[POOCH]

Hello,

I've inherited a customization of Campaign Response in which the Customer
attribute is read-only on the form (in both create and update modes), and I
can't figure out why. Neither the form nor attribute has any javascript, the
attribute "customer" (which is a partylist) is Searchable, and the
field-level behavior for the attribute on the form has "Field is read-only"
unchecked. The attribute is read-only even for the System Administrator, so
it's not an authorization issue.

What else could be making this field read-only?

Thanks!