Rob, sorry no-one got back to you. I did the Norte last year but on foot. However I met lots of cyclists. So much of the Norte is on tarmac that walkers and cyclists share the way a lot of the time. In Cantabria and Asturias the cycling contingent have been out in force, painting a lot of alternative yellow markers for cyclists labelled 'bici' which sometimes confuses us walkers as well!
Skaggs - that first week in the Basque country is probably the one time where you do need significantly different routes to the walkers as there were several very steep very muddy sections. I'd have a look at the Cicerone guide for walkers, purely because it has decent maps which often show the road nearest to the walking path. Also I think the CSJ guides often add a paragraph on any cycle detours off the walking route, but I haven't checked whether their Norte route does this - someone else on here will be able to say...
cheers, tom