Both MaemoMapper and Roadmap import GPX based routes
Since Roadmap uses vector maps, onboard routing is doable. This is in fact the direction Roadmap is going. It already supports address lookups natively. But support for onboard offline routing is probably not going to be for real until 1.1.0k.(map format is likely to change a bit with the Census Data as it's in shapefile format. So routing will likely wait until the dust settles from that...)
On the bright side, openstreetmaps.org support is coming much sooner. Roadmap 1.1.0h supports maps built from openstreetmaps.org xml files. 1.1.0i will include the buildmap_osm binary and will be able to download data and build vector maps on the fly for anywhere that openstreetmaps.org has info. This means that users outside the U.S.A. will now be able to use Roadmap!