I've documented the private vocabulary URI space that exists at vocab.org. If you need a safe URI for a namespace or a set of schema terms that you think might be around for a long time and you might want HTTP dereferenceablity then this is the place for you. Even if you don't think they'll be around for a long time it might be worth considering. Experimental vocabularies have a habit of outlasting their creator's original vision and with the types of data retention policies made possible by super-cheap storage it might be nice to have working URIs in 5, 10 or 20 years time.

Note to self: must find the time for improving the upload and maintenance of schemas at vocab.org