And the other cause is people meddling with the fuelling screw on the pump in the belief that more fuel =more power.......what they fail to understand though is that with a diesel you need more air to go with it!!
if it's just started occuring though, it is likely to be leaking injectors: my Citroen XUD engine started to get smokey at around 200000 miles- i took the injectors out and the faces had recessed slightly around the needle tips, causing them to leak- I got all four reconned for less than �100, and it now goes through the smoke test on fast-pass.