Would be useful, but I think it would be heavy program - but what you are saying is that one person could read a message with words censored out, and another would read the same message with all the swear words included.
I am not sure that it is a good idea having it set at board level as either a site would be set up to censor out such words, or it wouldn't - if one board had it off then so would the whole board, as the same people would either tolerate it or not, on any forum. But at a user level, it could be interesting - I guess guests would automatically have censoring on.
Ian.