[UPDATE: I'm stuck at a conference, but Paul Richfield (aka, Mr. Anti-blogger) tells me that Northrop Grumman is telling inquirers today that they are "not at liberty" to discuss Green Dart. (see "comments" link.) The plot thickens ...]
Whatever the "Hunter Green Dart" is, Northrop Grumman is building one for the US Army for $22.5 million. (See yesterday's contract announcement below.) There's already a handful of RQ and MQ variants of the venerable Hunter UAS, so perhaps this is another one.
While we attempt to responsibly answser that question, does anyone want to try to speculate/guess/make-up an explanation?
Northrop Grumman Systems Corp., Sierra Vista, Ariz., was awarded on Sept. 28, 2007, a $22,499,406 modification cost-plus-fixed-fee contract for the Hunter Green Dart. Work will be performed in Sierra Vista, Ariz., and is expected to be completed by July 31, 2008. Contract funds will not expire at the end of the current fiscal year. This was a sole source contract initiated on Aug. 31, 2007. The U.S. Army Aviation and Missile Command, Redstone Arsenal, Ala., is the contracting activity (W31P4Q-04-C-0082).

Steve,
Just got off the phone with N-G; they "are not at liberty to discuss Green Dart" at all, and advised me to contact INSCOM.
Cheers,
Paul
Could it be somehow connected with the latest Viper Strike contract award – just guessing?
Perhaps an EW variant of the Hunter?
It could be an EW variant, but it would have to be an awfully small low-band transmitter to fit on a Hunter. The contract to Northrop also would seem to rule out the only UAV EW payload candidate that I'm aware of: the BAE Systems Adaptive Joint Communications Node (AJCN). But maybe Northrop has a mini-jammer payload up their sleeves as well.