You can use Maemo Mapper's map downloading functionality to download maps from a repository (like Google Maps, although there may be end-user license restrictions on this) onto a memory card. Otherwise, Maemo Mapper can download maps as needed (using "Auto-Download"). Both require an internet connection, of course, but the former allows you to download maps "in advance" and view them without an internet connection. (The latter still saves the maps to disk for later access offline, but you need to be online for the initial download.)