MB has never officially acknowledged a problem with the engine, but they do fail (bent rods) in normal service. The rods have been re-designed and are now much heavier than previously.
Some people have had good luck getting the engine replaced under warranty, but if you are out of warranty, it can be a large pain to get any statisfaction. New engine is EXPENSIVE, even a short block can be prohibitive.
There is no official program for repair or replacement, either.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!