I don't know whether this is a real bug but I think Zend_Validate::is() with own namespaces is working by a fluke.

Zend_Validate::is(123, 'Int', array(), array('My_Validate'));

// My/Validate/Int.php
// Zend/Validate/My/Validate/Int.php

Zend_Validate::is(123, 'Int', array(), array('My_Validate'));

// My/Validate/Int.php

Index: library/Zend/Validate.php
===================================================================
--- library/Zend/Validate.php   (revision 21290)
+++ library/Zend/Validate.php   (working copy)
@@ -204,7 +204,7 @@
                     $file  = str_replace('_', DIRECTORY_SEPARATOR, $class) . '.php';
                     if (Zend_Loader::isReadable($file)) {
                         Zend_Loader::loadClass($class);
-                        $className = $class;
+                        $className = $class; break;
                     }
                 }
             }

This patch is not breaking any unit tests.