I was recently told that the primary reason that the ULSD fuel is required for most of the '07 vehicals is that their exhaust system is equipped with a "particulate" trap to limit the amount of soot and any other debris they emit.

The LSD fuel has more sulfur than the trap can handle over extended use.
Trap can clog and cause restriction.

The '07 5.9 CTD is one exception to the rule (no trap) so LSD is O.K.
The 6.7 cummins and the 3.0 engine that will be in the Grand Cherokee (early '07) have the trap.

Gary.