I've discovered relations and am using them to map some of the public bus routes in my city, Dublin, Ireland.

I'm mapping them like this:
* Bus stops are nodes, with highway=bus_stop and operator="Dublin Bus".
* The relations are type=route route=bus ref=$BUSNUMBER operator="Dublin Bus"
* I'm then adding the bus stop nodes to the relation. Sometimes I make them have the 'stop' role, but I think it should be obvious when a node is a member of a bus route and it's a highway=bus_stop it should be obvious. :)
* I'm adding the ways. No need to mark them with a certain role. If a bus route splits, (because of oneway restrictions), I'm just adding them. It should be obvious when this happens (the route splits in 2 along two opposite oneway streets, then meets up later).

Any thoughts? It might be fun later to figure out how to get mapnik to render these as a separate later.