There has been a lot of discussion in the talk mailing list recently about contours. These do not feature in the base OSM maps but appear in the cycle map, Freemap and the skiing piste map. the usual approach seems to be to create shape files and draw contours as lines, but thre are two drawbacks: there is a lot of extra data to store, download and render, and contours do not directly indicate elevation unless the height figures are displayed or users can work out which are high and which are low areas from extra information such as rivers.
I prefer an alternative approach of incorporating elevational information as a coloured relief map background to OSM bitmap tiles. There would be a small increase in files sizes but not enough to significantly affect download or rendering times, while the coloured bands are easier to interpret than simple contour lines.
I have described this proposal in more detail on the relief maps page.