<iframe width='480' height='350' frameborder='0' src='http://spreadsheets.google.com/pub?key=trN2LJ1ZLa6cX3ug7RRpDFQ&output=html&widget=true'></iframe>

The width and height parameters don’t scale your spreadsheet as they would with an embedded <img>; they just control how much of it is visible at one time. If it’s wider (as mine is) or taller than the given values, scrollbars will appear. The src parameter, of course, refers to the spreadsheet itself. You get this entire block of code from the “Publish to the web” dialog that’s accessed via the “Share” button on the spreadsheet.