@MetalGearSolid: The current packages don't include the gosmore plugin as I had the impression people wouldn't use it since no ready-made maps are available. When compiling and installing Marble in scratchbox using the armel target, you get the plugin and can copy it to the N900.

@gadgetChris: Very infrequently the monav routing daemon reaches a state where it does not work anymore and it needs to be killed manually. An easy way to do that is to reboot the N900. Does that help?
Have you checked the routing configuration in Marble? It does some basic sanity checks for Monav as well (checks whether the daemon can be executed and whether at least one map is installed).