With MOSS 2007 User Profile Properties, in most cases you're allowed to set Policy Settings as to whether the property is required, who can see it and whether or not the user can change this policy. However there are apparently some of these properties that are considered reserved by SharePoint and the options to change the policy settings are grayed out. But where there's a will (and a lack of fear from poking at sharepoint databases), there's a way to adjust these settings.
They key to this is in the UserPrivacyPolicy table in your Shared Services database. First you must find the PropertyID for the property you want to unlock in the PropertyList table. Once you have this, find the record for this property and set the value of the IsPolicyOverridable field to True. Now when you edit the property settings via the normal central administration web interface, you should be able to override the default settings.
Disclaimer: Making changes to your SharePoint database directly could be dangerous to the stability of your install and be undone by future upgrades. Do this at your own risk.