I've reviewed a new application called Ultra Recall. It's a tree-based information manager with some unique features. I haven't really put it through its paces in terms of managing GTD lists. I'm fairly certain it would be easy to do (a context list could be added as an item attribute, and saved searches could pull together different context tasks), but it may be useful for reference type material in any case.
Note that I spend 99.9% of my work time in front of my primary work computer, so Palm approaches are not that important to me. I mainly use the Palm for input (meeting notes).
My initial reactions are positive, and it seems to be one of the stronger contenders in the tree-based approach.
Ultra Recall Review