I found this small PHP script by Reinhold Weber which can quickly compress multiple CSS files to one compressed CSS file.

header('Content-type: text/css');
 
ob_start("compress");
 
function compress($buffer) {
    /* remove comments */
    $buffer = preg_replace('!/\*[^*]*\*+([^/][^*]*\*+)*/!', '', $buffer);
    /* remove tabs, spaces, newlines, etc. */
    $buffer = str_replace(array("\r\n", "\r", "\n", "\t", '  ', '    ', '    '), '', $buffer);
    return $buffer;
}
 
/* your css files */
include('screen.css');
include('typography.css');
include('grid.css');
include('print.css');
include('handheld.css');
 
ob_end_flush();