lessphp for WordPress: filter for stylesheet URLs so that if there is a corresponding file with a .less extension and that file is newer than this .css file, then lessphp is invoked to rebuild the stylesheet: the theme comment header is preserved.

wordpress-lesscss.php

<?php
/**
 * Filter stylesheet URLs so that if there is a corresponding file with
 * a .less extension and that file is newer than this .css file, then lessphp
 * <http://leafo.net/lessphp/> is invoked to rebuild the stylesheet: the
 * comment header is preserved in the LESS output. Requires that lessphp be
 * located in <TEMPLATEPATH/lessphp/lessc.inc.php> This filter may be invoked
 * in header.php as follows:
 * <link rel="stylesheet"
 *       href="<?php echo apply_filters('style_loader_src', get_stylesheet_uri(), null); ?>"
 *       type="text/css"
 *       media="screen" />
 * Filter also includes filemtime() in query string so that far-future expires
 * may be employed.
 */
function less_filter_style_loader_src($src, $handle = null){
    $parsedSrc = parse_url($src);
	//Process this file if the src's host is the same as this host (or if it's not specified)
	if(!@$parsedSrc['host'] || @$parsedSrc['host'] == parse_url(get_option('siteurl'), PHP_URL_HOST)){
		//Get the paths to the .css and .less files
		$cssPath = trailingslashit(ABSPATH) . $parsedSrc['path'];
		$lessPath = preg_replace('/\.css$/i', '.less', $cssPath);
		
		//Only process the file if a corresponding .less file exists and if the
		// .css file does not or it is older than the .less file
		if($lessPath && file_exists($lessPath) &&
		   (!file_exists($cssPath) || filemtime($lessPath) > filemtime($cssPath)) &&
		   include(TEMPLATEPATH . '/lessphp/lessc.inc.php')
		){
			try {
				$lc = new lessc();
				$lc->importDir = TEMPLATEPATH . '/';
				$lessCode = file_get_contents($lessPath);
				$cssCode = $lc->parse($lessCode);
				
				//Get the comment header out of the source .less file to preserve it
				$header = '';
				if(preg_match('{(^(?:\s*/\*.+?\*/\s*)+)}s', $lessCode, $matches))
					$header = $matches[1];
				
				//Add a warning to the header to instruct people to not modify it
				// directly since it is dynamically generated by lessphp.
				$header .= "\n/*\n";
				$header .= wordwrap("WARNING! This file is automatically generated from '" . basename($lessPath) . "' by lessphp."
				 . "To make modifications to the stylesheet, modify that file simply reload"
				 . "the page to be automatically regenerated by WordPress (requires that lessphp be available: http://github.com/cloudhead/less)", 80, "\n"); //\r\n
				$header .= "\n*/\n";
				
				//Write out new .css file
				if(!@file_put_contents($cssPath, $header . $cssCode))
					throw new Exception("Unable to write to " . basename($cssPath));
			}
			catch(Exception $e){
				//Construct an error message to display in the page
				$error  = "<p style='color:red; padding:0.5em; border:solid 1px red; background:white; position:fixed; top:0; left:0;'>";
				$error .= "<b><a href='http://github.com/leafo/lessphp' target='_blank'>lessphp</a> error in ";
				$error .= "<a href='" . esc_attr(preg_replace('/.css$/', '.less', $src)) . "'>";
				$error .= basename($lessPath) . "</a></b>: " . esc_attr($e->getMessage()) . "</p>";
				add_action(
					'wp_footer',
					create_function(
						'',
						"print " . var_export($error, true) . ";"
					)
				);
			}
		}
		//Append the filemtime to the CSS file so that far-future expires can be set
        $src = $parsedSrc['path'] . '?' . @filemtime($cssPath);
    }
	return $src;
}
if(!is_admin())
	add_filter('style_loader_src', 'less_filter_style_loader_src', 10, 2);