I've been trying to remember my own first encounters with the yarn. I think it was an audio tape version of the story - we had one of Tom Sawyer and one of Huckleberry Finn. I don't think either of these could have been more than forty minutes or an hour long, so a Hell of a lot of condensation and editing must have taken place.
I do recall, however, on reading the book later on, being startled not just at how much of the plot had been redacted in my childhood version, but also at the frequent appearance of the word nigger. While I can't be sure, this suggests that it had also been cut from the taped version I grew up with; though I do remember the beastly Injun Joe in the accompanying adaptation of Tom Sawyer, in all his politically incorrect glory.
According to the Daily Mail (2) (hardly the best source, but the one I have conveniently to hand), the use of 'nigger' in the text has lead to the novel being the "the fourth most banned book in U.S. schools." How many schools have actually banned it is less clear; though the lack of any number to materialize may indicate it is not great. I suspect the problem is more that it simply fades into the background, left untaught and unmentioned.
As for the actual process of substitution itself, I've been wondering if slave isn't more suggestive of the 19th century context than nigger is. The word probably seems worse, more demeaning and more insulting to us than it did to Twain and his contemporaries. So in that sense, we all know a slave is a possession, not a full person. Nigger, it seems, is something far worse now. So in that sense, Gribben may be retaining Twain's sense, if not his actual words.
It isn't being suggested that this should be the only edition available - the original and the expurgated version will exist side by side. The difference will be that students will be exposed to it. I doubt a great number of people who haven't encountered the tale as children feel a sudden urge to read it as adults; some, perhaps, but not a lot. Whereas those that have some familiarity with the book from childhood are more likely to revisit it, as I did.
1 - "The Adventures of Huckleberry Finn: controversy," wikipedia article relating to the novel by Mark Twain. Viewed 8th of January, 2010. (http://en.wikipedia.org/wiki/Adventures_of_Huckleberry_Finn#Controversy)
2 - "Political correctness takes on a great American novel: Huckleberry Finn removes N-word," by Tom Leonard. Published in the Daily Mail, 5th of January, 2010. (http://www.dailymail.co.uk/news/article-1344192/Huckleberry-Finn-removes-N-word-Political-correctness-takes-Mark-Twains-classic.html)