Personnally I still carry my Zaurus for PIM duties, for two reasons : first, the keyboard makes it easier and quicker to take down meetings etc. on the fly ; and of course it has Ko/pi (korganiser, platform-independent) and the rest of the kdepim/pi suite (address book, mail client, password safe etc.).
Ko/pi really shines as a mobile agenda/todo. It probably has more interface options and features than most of the commercial offerings on the various PDA platforms out there.
If you were to start on a maemo-specific solution I would say that porting ko/pi would be the sensible thing to do (rather than starting from scratch etc.). Unfortunately, as the initial "K" shows, the suite has evolved from a KDE-specific project and is probably tightly bound to (at least) Qt and some KDE libs, so porting it to a Gtk-based environment like Maemo is probably not trivial at all, sadly.