You could also simply download the plugin I wrote for this as well. Although I have submitted it to WordPress.org they have not posted it yet so here is a link in the mean time. Please note the plugin is a little more customizable then the code below.

-Auto Keywords and Description Generator-

//Code

<?php
$keywords = "";
$description = "";

if(is_home() || is_front_page())
{
	//LOAD KEYWORDS FROM USER SPECIFIED STRING
	$keywords = "Insert your homepage keywords here";

	//LOAD DESCRIPTION FROM BLOGINFO
	$description = bloginfo('description');
}
elseif(is_single())
{
	//LOAD POST TITLE AS DESCRIPTION
	$description = $post->post_title;

	//BUILD LIST OF KEYWORDS FROM TAGS
	$tags = wp_get_post_tags($post->ID);
	foreach($tags as $tag)
		$keywords = $keywords . $tag->name . ", ";
}
elseif(is_category())
{
	//LOAD CATEGORY DESCRIPTION
	$keywords = single_cat_title('', false);
	$description = category_description();
}
?>
<meta name="keywords" content="<?php echo $keywords ?>" />
<meta name="description" content="<?php echo $description ?>" />