Update summer 2008: For more modern Maya versions, most of these attributes have been incorporated into the UI. To see the latest few still left to add manually, use the Maya search function in the Maya Help, and look for "Dynamic Attributes". It should be the top hit.

The mental ray for Maya plugin understands some extra attributes that are not explicitly connected to the UI in Maya 6. When created as dynamic attributes on the appropriate nodes, so as to appear in the UI as extra attributes, these undocumented attributes extend the controllability of features in mental ray. Please understand that because these attributes are "under the hood", they are unsupported and we cannot guarantee whether or not, or in what form, each may appear in future Maya releases. In fact, Maya 6 is old enough, that we are removing this reference, as most of it is now in the Maya Help reference.