This howto has been superseded by http://drupal.org/node/815816 (patch for Upload preview module).
Tested on:

• Drupal 5.x
• Content Templates module
• ImageMagick 6.3.7
• GhostScript 8.15.3

Using ImageMagick + GhostScript you can convert the first page of a PDF into a thumbnail image linking to file. You can cut and paste this function on your theme main script: you have to manually create the thumb directory and grant write permission by scripts. Check also the path to ImageMagick convert command.

function pdf_thumb_attachments(){
 $allowed_mime = array("application/pdf");
	foreach($files as $file){
	if(in_array(strtolower($file->filemime),$allowed_mime) && strstr(strtolower($file->description),"classifica")){
		$title = $file->description;
		if(substr($title,-4)==".pdf"){
			# create link title from file name (Transliteration module suggested)
			$title = str_replace("_"," ",substr($title,0,strlen($title)-4));
		}
		$img = "";
		$local_src_path = getcwd() . "/" . $file->filepath;
		$destfilename = substr($file->filename,0,strlen($file->filename)-4) . ".gif";
		# YOUR-FILES-DIRECTORY/pdfgen will hosts the generated thumbnails
		$local_dest_path = getcwd() . "/" . file_directory_path() . "/pdfgen/" . $destfilename;
		$imageurl = base_path() . file_directory_path() . "/pdfgen/" . $destfilename;
		/* create thumbnail on node reading if file doesn't exist */
		if(!file_exists($local_dest_path)){
			# choose the first page of the PDF, scale to 90x90px and convert to GIF
			$exec = '/usr/bin/convert -scale 90x90 "'.$local_src_path.'"[0] "'.$local_dest_path . '"';
			shell_exec($exec);
			/* delete this line if you cannot read the generated file
			$exec = "chmod 777 ".$local_src_path;
			shell_exec($exec);
			//*/
		}
		$img = '<img src="'.$imageurl.'" alt="'.$title.'" />';
		$output .= "<li>". l($img." ".$title,$file->filepath, $attributes = array("title"=>$title), $query = NULL, $fragment = NULL, $absolute = TRUE, $html = TRUE) . "</li>";
	}
	}
 if(!empty($output)){
	$output = '<div class="my-attachments"><ul>' . $output . "</ul></div>";
 }
 return $output;
}

Update

You can alternatively use imagemagick function provided by image.module (the image.imagemagick.inc file on drupal/includes):

function file_preview_path($filename, $filepath = 'filepreview') {
  /** generate jpg thumbnails **/
  return $filepath . "/" . substr($filename,0,strlen($filename)-4) . ".jpg";
}

 function file_preview(&$file, $pages = Array(1)) {
   $allowed_mime = array("application/pdf");
   if(in_array(strtolower($file->filemime),$allowed_mime)){
	$local_src_path = getcwd() . "/" . $file->filepath;
	$local_dest_path = getcwd() . "/" . file_directory_path() . "/" . file_preview_path($file->filename);
	# create thumbnail only when needed
	$imagemagick = getcwd() . '/includes/' . 'image.imagemagick.inc';
	if(!file_exists($local_dest_path) && file_exists($imagemagick)){
		include_once($imagemagick);
               _image_imagemagick_convert($local_src_path.'[0]',
                                                       $local_dest_path, array('-colorspace RGB'));
		# all can read thumbnail files
		if(file_exists($local_dest_path))
			chmod($local_dest_path, 0777);
	}
	return file_preview_path($file->filename);
    }
    else{
	return FALSE;
    }
 }