The ACM SIG documentclasses interact badly with the use of the geometry package for getting the paper size right. They also print very badly when in A4 papersize, sent to letter paper. To solve this problem, put this in your LaTeX preamble. (You should probably remove it before submitting the paper electronically to the conference)

\documentclass{sig-alternate}

%remove these lines before submitting
\usepackage{ifpdf}
\ifpdf
\setlength{\pdfpageheight}{11in}
\setlength{\pdfpagewidth}{8.5in}
\else
\special{papersize=8.5in,11in}
\fi

This handles both LaTeX with the dvips driver, and pdfLaTeX.