In WordPress, takes a url from a custom field and turns it into a thumbnail ID. Found here: http://pastie.org/2386814

gistfile1.php

<?php
/*
Plugin Name: Update thumbnails
Plugin URI: http://pmg.co
Description: Takes a url from a custom field and turns it into a thumbnail ID
Version: .01
Author: Christopher Davis
Author URI: http://pmg.co/people/chris
*/

/**
* CHANGE THESE!
*/
define( 'OLD_THUMB_KEY', 'your_thumb_key' );
define( 'DELETE_OLD_THUMB', false );

/**
* Uncomment the next line to run the above function on plugin activation.
*/
//register_activation_hook( __FILE__, 'wpse26138_set_new_thumbs' );

/* Stop editing here
-----------------------------------------*/
function wpse26138_set_new_thumbs()
{	
	// get an array of all the posts
	$posts = get_posts(
		array( 
			'numberposts'	=> -1,
			'post_type'		=> 'post'
		)
	);
	
	// bail if our get_posts() call failed
	if( empty( $posts ) ) return;
	
	// Loop through the posts
	foreach( $posts as $p )
	{
		$image_url = get_post_meta( $p->ID, OLD_THUMB_KEY, true );
		
		// no thumbnail? on to the next
		if( empty( $image_url ) ) continue;
		
		// find our mime type for later
		$filetype = wp_check_filetype( $image_url );
		
		// Set up an array of args for our new attachment
		$args = array(
			'post_mime_type' => $filetype['type'],
			'post_title'	 => esc_attr( $p->post_title ), // you may want something different here
			'post_content'	 => '',
			'post_status'	 => 'inherit'
		);
		
		// Insert the attachment!
		$thumb_id = wp_insert_attachment( $args, $image_url,  $p->ID );
		
		// gotta set up some meta data (height, width, etc)
		// the functions are in this file, so we have to include it
		require_once(ABSPATH . 'wp-admin/includes/image.php');
		$metadata = wp_generate_attachment_metadata( $thumb_id, $image_url );
		wp_update_attachment_metadata( $thumb_id, $metadata );
		
		// Finally! set our post thumbnail
		update_post_meta( $p->ID, '_thumbnail_id', $thumb_id );
		
		if( DELETE_OLD_THUMB )
			delete_post_meta( $p->ID, OLD_THUMB_KEY );
	}
}