Though only a mere rumour at this point, it's a damn interesting one. In an interview with Robert Hillburn of the L.A. Times concerning the future of Rage Against the Machine, Hillburn states:
I'm eager to hear what Rage/Cornell came up with, but I'm also intrigued that Zack and Trent Reznor are working on some stuff together. Keep your fingers crossed. That could be amazing--though its hard to imagine such strong personalities finding a common ground musically.
Remember, this is a completely ungrounded rumour.