A new Gmail Labs "Apps Search" feature extends Gmail's search functionality; "When enabled, you'll see a list of included search results from Google Docs and Google Sites below Gmail's own search results." If you want to search Google Docs and Google Sites, without displaying Gmail results, add -type:mail to your query. Also, after enabling Apps Search, Gmail will show a "did you mean" message if your query includes misspellings.
Most likely, Google will let us search all the data stored in a Google account 'Gmail messages, Calendar events, Google Reader subscriptions, Blogger posts, Picasa Web photos, contacts, tasks, Google Voice history and more' from a single search box.
For some reason, Apps Search has a bug and returns incorrect URLs for Google Docs results.