Allow Users to Set Field Level Permissions

Description - Our users would like to be able to 'lock' fields on projects and make them only editable by certain users. Right now there doesn't appear to be any functionality that accomplishes this. For example, the PM may want to 'lock' the Duration field so only specified users can update that field. Right now, anyone with Contribute or Manage access to the project is able to adjust those values. Or anyone with View access that has a Task level assignment. These permissions vary from project to project so we would like to be able to set these permissions on the object itself, and not as an Enterprise level rule.

Why is this feature important to you - This is important for data integrity. Not all users should have access to edit all fields just because they have access to a project. 

How would you like the feature to work - I envision some type of 'lock' or 'share' button on the field that allows us to set permissions for that particular field. This would supersede any Project level permissions. Ie: User John has Contribute access to the Project but only has View access to the 'Duration' field.