translucency problem

[ior=0]

Hi I have a translucent material that looks ok (or as I would expect the mat to render) in RT, but when I do a production render I'm not getting the translucency I'd expect. See the attached image for example and setting.
Any help much appreciated...I have a feeling it is something really silly that I've forgotten.

[ior=0]

Brain isn't working - should RT be able to show fog for refractive mats. Cos if I turn off translucency and render I get the same as the RT result?

[zeronove]

that's was my first suspect, it seems to me remembering it's still unsupported in rt