I guess it's long been an issue that searching on SO is not perfect. I've just come a-cropper here by asking an exact duplicate because I searched for profanity and swearing instead of expletive and salty. (Following which I got quite comprehensively downvoted).
This would have been picked up if some kind of thesaurus-o-tron was at work on search. I don't know much about search and indexing but I guess that this kind of thing must be possible (i.e. to identify key nouns in a post and index that post by their synonyms as well).
EDIT; whilst I accept that Google is a valid answer to this question, I have seen Joel himself say that Google does not do search properly on Q&A-like content (e.g. by giving prominence to older content) and I have also seen him say that on SO "the act of asking a question is also the act of searching for the answer". If this is the case, and if I'm to end up losing a load of rep because the search algo could be improved, I'd like to see the search algo improved!