After using the PDF viewer from Google Docs for previewing PDF search results, Google started to enhance the cached version of XLS results with links to Google Docs. If you search for Excel files and click on "view as HTML" next to one of the results, you'll see three new options: a download link, a link to the HTML version and an option to edit the spreadsheet online using Google Docs.
I also found a quick way to view an Excel file in Google Docs:
(replace URL_OF_THE_XLS_FILE with the actual URL)
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