hmm - thanks steve - i tried various combinations, adding it to displacement mat. under shading group attributes and to displacement shader under mental ray on both the sss and the blinn, but still no luck.
i've been using the regular displacement node under create maya nodes, cos i didn't see anything under the create mental ray nodes - would that have something to do with it?
ah! - it's 4 AM here so i should probably get some sleep on it and have another look tomorrow. thanks again.