Until every search engine dynamically underlines and corrects misspelled word in the search box, people will continue make mistakes when searching. This make it very easy to get top ranking on misspelled words that apply to your web site i.e. more hits. (thus leading to more sales).
Now you don't want to give non professional image to our customers, but you can achieve this by placing these words is say the page title or directory, as most people don't look at title of the page or where the page stored.
To obtain a list of common misspelled words, one can perform some searches on this web sites:
http://www.wsu.edu/~brians/errors/errors.html
http://www.searchspell.com/typo/
http://www.barnsdle.demon.co.uk/spell/error.html
http://directory.google.com/Top/Science/Social_Sciences/Language_and_Linguistics/Natural_Languages/Germanic_Languages/English/Spelling_Reform/Problems_in_Traditional_Spelling/