From pickuptruck.com,

The 4500/5500 Cummins' exhaust line traps and burns off soot like the Ram 2500/3500 Heavy Duty pickups, but the diesel particulate filter is mounted underneath the cab in the Class 4/5 trucks instead of under the bed in the HD Rams. And while the 2500/3500 HD pickups meet 2010 emissions standards for Nitrogen Oxide (NOX) today, using Cummins' near maintenance-free Adsorber catalyst, the 4500/3500 trucks will rely on urea selective catalytic reduction (SCR) to scrub NOX molecules. The urea tank will be mounted on the chassis by 2010 and will require periodic refills.

Looks like urea is going to happen. Now I'm REALY glad that the pickup trucks already meet the 2010 emission laws. I also wonder if this means that Ford and Chevy will have to have a urea tank on there trucks since they don't meet the 2010 laws?