Basic example to produce the upload directory URL.

<?php $upload_dir = wp_upload_dir(); ?>
<?php echo $upload_dir['baseurl']; ?>

More in-depth break down of the data returned.

<?php
$uploads = wp_upload_dir(); // Array of key => value pairs
/*
	$uploads now contains something like the following (if successful)
	Array (
		[path] => C:pathtowordpress/wp-content/uploads/2010/05
		[url] => http://example.com/wp-content/uploads/2010/05
		[subdir] => /2010/05
		[basedir] => C:pathtowordpress/wp-content/uploads
		[baseurl] => http://example.com/wp-content/uploads
		[error] =>
	)
	// Descriptions
	[path] - base directory and sub directory or full path to upload directory.
	[url] - base url and sub directory or absolute URL to upload directory.
	[subdir] - sub directory if uploads use year/month folders option is on.
	[basedir] - path without subdir.
	[baseurl] - URL path without subdir.
	[error] - set to false.
*/

echo $uploads['path'] . '<br />';
echo $uploads['url'] . '<br />';
echo $uploads['subdir'] . '<br />';
echo $uploads['basedir'] . '<br />';
echo $uploads['baseurl'] . '<br />';
echo $uploads['error'] . '<br />';

$upload_dir = ( $uploads['url'] );
$upload_dir_alt = ( $uploads['baseurl'] . $uploads['subdir'] );

// Now echo the final result
echo $upload_dir . '<br />'; // Output - http://example.com/wp-content/uploads/2010/05

// Using year and month based folders, the below will be the same as the line above.
echo $upload_dir_alt . '<br />'; // Output - http://example.com/wp-content/uploads/2010/05
?>