There is a note: "Site-specific custom namespaces are generally assigned in the 100 to 199 range. I would recommend staying out of this range for anything to be defined by an extension."
These numbers are however used by Semantic MediaWiki and Semantic MediaWiki Extensions. It would indeed be very useful to have numbers reserved for site-specific namespaces. According to http://www.mediawiki.org/wiki/Extension_namespace_registration numbers from 500-599 are still free. Would'nt it be a good idea to really reserve this range for site-specific custom namespaces?