Awhile back I did a few posts about this subject and this morning I found an email from DerechEmet, who sends out various quotes, about the dangers of Nivul Peh. Sorry I have not posted in awhile; hopefully I will be posting more often in the future...thanks for reading, Mashiach Now!!
Sefer HaMidot, chapter Nivul Peh:
 A person who speaks vulgarity, it is certain
[literally, known] that his mind thinks evil thoughts.
 Through [the sin of] speaking obscene words,
[the speaker] comes to [the additional sin of speaking
forbidden kinds of] flattery.
 Through [the sin of] speaking obscenities [comes]
new sorrows and [evil] decrees, and the young men
of Israel die young, G_d forbid, and [even] orphans
and widows cry out and are not heard.
 He who defiles his mouth by speaking obscenities,
even if he had a sealed [Divine] decree of 70 good years,
they are transformed to [70 Divinely decreed] bad [years],
and Gehinom [Hell] is deepened for him.
This also applies to he who heard [his fellow Jew speak
obscenities] and he remained silent [instead of protesting
against the obscenities, if and when he was able to protest].
 [The sin of] speaking [foul language] is more
severe than [sins involving] physical actions.
Rabbi Nachman of Breslov was born in 1772 CE and died in 1810 CE.