I have a feeling that it has more to do with a lack of marketing and presence than the fact that it is using the clamshell design.
Palm is everywhere as far as marketing goes.
Meanwhile, we hear a peep out of Psion every once in a while. The Psion Advisor and I think that Psion is dipping because it hasn't introduced a new product in a long time. By the way, we're not really counting the Revo Plus as a new product. Also, the company isn't taking advantage of the marketshare it already has by supporting existing devices. I don't agree.