From Spencer Ackerman’s “latest post on TNR”:http://www.tnr.com/blog/iraqd?pid=2561.

bq. I don’t mean to suggest that Arab-Kurdish disputes over the multi-contested city [Kirkuk] are principally economic, but they’re not _not_ economic. (Emphasis in original.)

This kind of phrasing, with an apparently ineliminable double negation, is fairly common, especially when discussing vagueness. My opinion is that this is a kind of metalinguistic negation; the stress on the second _not_ is the same as the stress on _like_ in _I don’t like cricket; I love it_. What is being denied here is that it’s proper to say the disputes are not economic. In this case it is improper because it implicates that they are not at all economic. In the vagueness cases it would be because it implicates something is determinately true. E.g. of a borderline smart person you can say _He’s not *not* smart_, with strong stress on the second _not_ and some stress on _smart_, meaning that it would be wrong to say he’s not smart.