It looks like it's distributed under a CC-BY-SA licence. OSM actually changed from CC-BY-SA licence to ODbL because it didn't look like that CC licence is suitable for geo databases like OSM ( read more: http://www.osmfoundation.org/wiki/License/We_Are_Changing_Th... ).

It would have to be relicenced (which I presume would not be a problem since ODbL is spiritually very similar to CC-BY-SA). But as maxerikson mentions it's not clear that it should be in OSM