With gas prices on the rise again, and the fact that Nissan Titan sales have never really taken off, Nissan has apparently abandoned plans to create an even larger heavy-duty truck. The Titan platform also currently underpins the big Armada and Infiniti QX56 SUVs and the next generation of those trucks will likely migrate to a substantially lighter uni-body architecture. That would make them more crossover-like and more fuel efficient.
Since a heavy duty variant would have required a unique, substantially beefed up frame, in order to handle the increased payloads and towing capacity, the low sales projections just didn't justify the development. The lack of a suitable diesel to put in the the HD Titan surely didn't help either. The truck was due in 2009-10 as was a Toyota heavy duty truck that has also been shelved for now.