When I have a high traffic keyword or words, to gain extra traffic I also add misspellings like this:
if the word is "word", I also enter --
So I add every permutation that excludes 1 letter.
Then I also look at the Qwerty keyboard and for "word" I'd enter the letters immediately to the left and right of the letters, --
Now if any of these combinations are actually words that mean something completely different, I don't enter it, so in no way is this misleading the visitor. It's just an attempt to capture a large portion of misspellings.
I've goosed our AdWords traffic by at least 10% using this, because the click-throughs are much higher for the misspellings since they can't find what they were looking for. I know it's not remotely rocket science, but it's a way to put to a simple algorithm how people misspell words. It's actually pretty predictable and takes only a little bit more time.