Does Geoserver have a routing functionality for the OSM data in PostGIS?
has all the components for routing with Geoserver (is in development and in Alpha)
OpenTripPlanner is a project developed to do multimodal routing on top of GeoServer. It's better at transit, bike and walking than driving at the moment, but has a very powerful core routing engine. At some point the OpenGeo team hopes to make it a community module for GeoServer, so it can be more easily incorporated.