I think that Seth and Klas are right, I've tried exhaustively to create transparency with a reflective shader, no cigar.
I remember Walli posting an example here before with transparency from a reflective shader, I tried matching his settings on my own, still nothing. I put it down to Walli using a different unreleased version of Terragen that the general public did not have. After the new release, though, I tried again and had the same negative results.
I can easily create a fake reflective transparency effect, like this appears to be. I will try this in an imported model instead of a native TG sphere to remove 'double sided surface' manually but, since a TG sphere is single sided anyway, I'm not sure it'll make much difference.
I'm probably missing something really simple, if you have indeed made transparency here, Marc.