Input, Session and Cookie get's will return NULL.
Read more about this change here:
http://codeigniter.com/forums/viewthread/215833
diff --git a/system/core/Input.php b/system/core/Input.php
index e916ac6..97be9e6 100755
--- a/system/core/Input.php
+++ b/system/core/Input.php
@@ -135,7 +135,7 @@
 	{
 		if ( ! isset($array[$index]))
 		{
-			return FALSE;
+			return NULL;
 		}
 
 		if ($xss_clean === TRUE)
@@ -659,7 +659,7 @@
 
 		if ( ! isset($this->headers[$index]))
 		{
-			return FALSE;
+			return NULL;
 		}
 
 		return ($xss_clean === TRUE)