Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 18615)
+++ wp-includes/functions.php	(working copy)
@@ -2076,9 +2076,21 @@
  * @return bool Whether the path was created. True if path already exists.
  */
 function wp_mkdir_p( $target ) {
+	$wrapper = null;
+
+	// strip the protocol 
+	if( wp_is_stream($target) ) {
+		list( $wrapper, $target ) = explode( '://', $target, 2 );
+	}
+
 	// from php.net/mkdir user contributed notes
 	$target = str_replace( '//', '/', $target );
 
+	// put the wrapper back on the target
+	if( $wrapper !== null ) {
+		$target = $wrapper . '://' . $target;
+	}
+
 	// safe mode fails with a trailing slash under certain PHP versions.
 	$target = rtrim($target, '/'); // Use rtrim() instead of untrailingslashit to avoid formatting.php dependency.
 	if ( empty($target) )