geojson/draft-geojson — pull request #19 by sgillies (2014-02-21 17:47:16+00:00)
Title: Specify that Feature.id SHALL be a string.

As I said in https://github.com/mapbox/geojsonhint/issues/16, I think that string is absolutely the best type for Feature ids and that the I-D should say so and require it.