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."