Originally Posted by MartinK

but I'm not sure if I'll be able to build it for Sailfish OS and I have serious doubts of an Android build being doable.

Originally Posted by MartinK

Still, the main issues still stands: How to get the data for rendering ? AFAIK big Mapnik tileservers use a Postgis database to fetch geometry during rendering - that would be probably too much of an overkill for the N900 and mobile devices in general. So some other more simple/lightweight format/store is needed that can still be plumbed to Mapnik (and/or ideally other potential renderers). Raw OSM data files (XML, PBF) won't do for any non-trivial area due to missing support for efficient random access and area queries.

Therefore some sort of custom format with pre-processed data will be needed - my current favorite is reusing the already present support for sqlite tile database files, but storing tile geometry instead of tile images, with optional per tile compression. Using this method the tile rendering backend would very easily query the geometry for any given tile.
There also options for just generating "geometry tiles" for all layers we might want to render for maximal simplicity and ultra fast fetching (this would make the database bigger than the raw OSM data for the area) or storing just a single (or a couple) of map layers and combine/split them for the given zoom level (meaning the database would not be (much) bigger than the raw OSM data for the area.

In short - I'm still investigating how to best do it and piecemail working on the needed components - testing your Mapnik port, investigating the QtQuick 2 Canvas, improving the modRana data repository, planning improvement in the tile storage module for "geometry tiles" support, etc.

Also as for related development - Basil Semuonov (probably best know for running Open Repos) has apparently been working on a offline rendering and routing server (I might have written about it already a while ago). That would also help a lot - provided that:

1. he actually ever releases it (he seems to be generally really busy)
2. it can be ported to Maemo (you have used your magic to port Magic, so I'm sure this would be easy for you ;-) )

More or less instant offline maps as his solution seems to have some sort of internal database (solving the data issue), can import raw OSM data and provides a localhost tile server (very easy to use by current modRana codebase).