GPE Palmtop Environment is a handhelp shell which provides a complete environment of software components and applications for tasks such as personal information management (PIM), audio playback, email, and web browsing. It can be used as a FOSS alternative to various of PDA systems but can also be used on other distributions.
GPE is using X11 as its windowing system. See Handhelds, for detailed information about the current progress of GPE.
GPE seems to be a good UI for E-Ink and older devices.
ROX which has a purpose to be a lightweight desktop UI might be useful in porting GPE.