Filter to add drop caps to first letter of blog posts in WordPress. Add to your active theme's functions.php.

add_drop_caps.php

/*
	Drop cap first letter of each post.
*/
function add_drop_caps($content)
{
	global $post;
	
	//only posts
	if(!empty($post) && $post->post_type == "post")
	{
		//find first p tag with a letter following it
		$match = getMatches("/\<p\>[A-Z]/i", $content, true);				
		if(!empty($match))
		{
			$letter = str_replace("<p>", "", $match);
			$dropcap = '<p><span class="dropcap">' . $letter . '</span>';
			$content = str_replace_once($match, $dropcap, $content);
		}		
	}
	
	return $content;
}
add_filter('the_content', 'add_drop_caps', 30);
add_filter('the_excerpt', 'add_drop_caps', 30);

/*
	Helper for Getting RegExp Matches
*/
function getMatches($p, $s, $firstvalue = FALSE, $n = 0)
{
	$ok = preg_match_all($p, $s, $matches);		
		
	if(!$ok)
		return false;
	else
	{		
		if($firstvalue)
			return $matches[$n][0];
		else
			return $matches[$n];
	}
}

/*
	Replace a string once.
	From: http://php.net/manual/en/function.str-replace.php#86177
*/
function str_replace_once($search, $replace, $subject) {
    $firstChar = strpos($subject, $search);
    if($firstChar !== false) {
        $beforeStr = substr($subject,0,$firstChar);
        $afterStr = substr($subject, $firstChar + strlen($search));
        return $beforeStr.$replace.$afterStr;
    } else {
        return $subject;
    }
}

This is going to fail if a post starts with a quotation mark, newline, parenthesis, Spanish ¡ character, etc.