Henri Bergius has the info on how to run OpenStreetMap on for mapping on your Nokia N800. In fact, it's incredibly simple. Using MaemoMapper, just add
http://tile.openstreetmap.org/%0d/%d/%d.png. to your Map repositories.
This is nice in a couple of ways. First, MaemoMapper, and the underlying Maemo, are open-source applications. So it's nice to use open geodata. Second, using and caching Googlemap/Yahoo/et al. tiles is probably a violation of their Terms of Service, especially when you use them for realtime navigation. By using OpenStreetMap as the default mapping tile provider, MaemoMapper now provides a base functionality that is free for stringy ToS that could get it into trouble and question its existence.