This is the URL for M-B's BlueTec Diesel Technology site in which they
describe how it works. It's expected next year on the '07 (updated)
E-Class, V-6 CDI diesel.
Basically, diesels' exhaust must be rid of soot and NOx. A ceramic cat
can eliminate the soot but the NOx is the hard part. So this technology
injects urea (ammonia and water) into the exhaust and a catalyst
converts it and the NOx into N and H2O.
The website has an excellent BlueTec presentation (aimed at European
truck operators) for us diesel voyeurs.
In 1980 when I bought my 300SD, we received a North American directory
of diesel fuel stations, I guess in 2007 they'll hand out a similar
directory of urea stations - but on a CD for the car's Nav system!