A feature available in the Apps Search experiment is added to Gmail "spell checker for your queries". For example, if you search for [anounced], Gmail will show a message above the results: "did you mean announced". Gmail doesn't have a special query spell checker, so it gets the suggestions from Google Search.
Gmail's search feature is a lot less sophisticated than Google's Web search engine, which replaces some of your words with synonyms or similar terms and sorts the results by relevance. Google could use the importance flags and other signals to find results that are likely to be important, the search engine could index attachments and it could become a module from a more comprehensive search engine that shows results from Gmail, Google Calendar, Google Docs, Google Reader, Google Maps, Google Buzz, Google Contacts and much more.