Spell Checkers normally are not that smart. They consider a word to be incorrect if they can’t find it in their dictionary. Even if words are in the dictionary, they still cannot identify the error of using the word in an improper context. Well Google seems to have realized that. Google Wave, the latest sensation from Google previewed recently at Google I/O conference, contains a context sensitive spell checker. To solve the above mentioned problem, the smart spell checker of Google Wave uses the language models built for Google Translate to search for words that don’t belong in a certain context.
Context sensitive spell checker is not a simple thing at the only context sensitive spell checker that works for me as a dyslectic is Ghotit.com
You may wish to look at the comparison of Google Wave, MS-Word-2007 and Ghotit spelling and grammar checker.
http://www.ghotit.com/contextspellcheckers.shtml
Google is a room for improvement.