All indications are that Ford will be dumping the Navistar diesels it's used in its Super Duty pickups almost forever in favor of an in-house-developed diesel sometime around the end of the decade. Apparently undaunted by the quality issues that Ford has repeatedly experienced with the Navistar engines, Nissan appears ready to step into the breach and pick up the supply for the Titan pickup.
Since it appears that Nissan won't be developing a heavy duty version of the Titan, it's not clear at this point if it will use the current 6.4L V-8 that Ford uses or the smaller engine that Navistar had developed for the light-duty Fords. Most likely it will be the latter engine, and it may be available for 2009. Nissan, as expected, won't comment on future products -- except when they do (they previously announced a diesel Maxima for 2010).
UPDATE: Mike Levine at Pickuptruck.com spoke with Nissan spokesperson John Schilling who told him, "We haven't announced anything about a diesel engine for the Titan. This news is speculation at this point, but we are continuing to study this technology for potential future use. The only vehicle we've said that will have a diesel engine is the Maxima in 2010."