http://www.autoblog.com/2008/10/20/new-diesels-wont-run-when-the-urea-tanks-is-dry/

Owners of the new crop of clean diesels will have another maintenance item to concern themselves with if their new cars are equipped with urea injection. The EPA requires that all diesels meet strict new emissions standards, which almost always requires the use of either a NOx trap or urea injection. To ensure that the system is working, a sensor checks both the level and the quality of the urea solution and will keep the car from starting if the tank is empty. The new Mercedes-Benz BlueTEC vehicles do. According to Mercedes, a counter will appear on the dash when there are twenty starts remaining. If the driver ignores the message, the car will not operate until at least two gallons of urea solution is added to the tamper-proof tank that resides in the spare tire well.

Engineers have designed the urea tanks to be large enough to last about 15,000 miles. This should be large enough that drivers never run out, as the tanks will be checked at every schedules service. As an alternative, small bottles of the urea solution will be available for owners who want to maintain the system themselves. The cost of the urea solution, which MB refers to as AdBlue, will mirror the cost of diesel fuel, so a refill shouldn't cost more than $30 or so. Before anybody asks, yes - urea is a component of urine, and no - peeing in the tank will not fool the sensors. You've been warned.