Yep, that is essentially what gnuite suggested, and it would work just fine for a small number of categories. But where you have a lot of categories (e.g. 60), that would involve:

(1) dividing your POI dataset into 60 separate files
(2) manually creating 60 new categories in Maemo Mapper
(3) separately importing each of 60 POI files (and making sure you don't screw up by accidentally specifying the wrong default category each time)

This kind of task is what automation was made for!!! That said, it is hardly a major issue, esp. since there is a workaround. But it would be nice to see it handled one day.