I have a client who wants to be able to embed an Excel document (one that is currently sitting on the same server as the HTML document) like how you would embed a Flash app on a
You are correct - at least, it can't be done cross-browser. You should just link to the Excel spreadsheet file.
Alternatively, if you only need to display the data, I'm sure you can find a PHP library that will read Excel files and display their contents in a HTML table.