Greenwald's follow-up is already finished and is available on DVD next week (it is also receiving his usual scattered and limited theatrical screenings). Titled Iraq for Sale: The War Profiteers, the doc focuses on the private corporations charging exuberant prices for goods in Iraq (a six-pack of Coca-Cola for $45?), much of which is, in the end, paid for by American taxpayers. Among these corporations, Haliburton is obviously given great attention, and the company has already spoken out against the film calling it, "a theory in search of a conspiracy."
Personally, I keep hoping that Greenwald will deliver a follow-up to Xanadu (at least the soundtrack, anyway), but as I'm in the minority there, I'm glad that fans of his other films can keep getting more and more from him.