Index: tests/Zend/Log/Formatter/XmlTest.php
===================================================================
--- tests/Zend/Log/Formatter/XmlTest.php	(revision 18428)
+++ tests/Zend/Log/Formatter/XmlTest.php	(working copy)
@@ -46,22 +46,30 @@
         $this->assertContains('foo', $line);
         $this->assertContains((string)42, $line);
     }
-    
+
     public function testConfiguringElementMapping()
     {
         $f = new Zend_Log_Formatter_Xml('log', array('foo' => 'bar'));
         $line = $f->format(array('bar' => 'baz'));
         $this->assertContains('<log><foo>baz</foo></log>', $line);
     }
-    
+
+    public function testCallbacks()
+    {
+        $f      = new Zend_Log_Formatter_Xml();
+        $fields = array('custom' => array('Zend_Log_Formatter_XmlTest_TestObject3', 'getValue'));
+        $line   = $f->format($fields);
+        $this->assertContains('<logEntry><custom>Custom value</custom></logEntry>', $line);
+    }
+
     public function testXmlDeclarationIsStripped()
     {
         $f = new Zend_Log_Formatter_Xml();
         $line = $f->format(array('message' => 'foo', 'priority' => 42));
-        
+
         $this->assertNotContains('<\?xml version=', $line);
     }
-    
+
     public function testXmlValidates()
     {
         $f = new Zend_Log_Formatter_Xml();
@@ -96,3 +104,11 @@
         $this->assertContains('&amp;amp', $line);
     }
 }
+
+class Zend_Log_Formatter_XmlTest_TestObject3
+{
+    public static function getValue()
+    {
+        return 'Custom value';
+    }
+}
\ No newline at end of file
Index: tests/Zend/Log/Formatter/SimpleTest.php
===================================================================
--- tests/Zend/Log/Formatter/SimpleTest.php	(revision 18428)
+++ tests/Zend/Log/Formatter/SimpleTest.php	(working copy)
@@ -46,14 +46,14 @@
             $this->assertRegExp('/must be a string/i', $e->getMessage());
         }
     }
-    
+
     public function testDefaultFormat()
     {
         $fields = array('timestamp'    => 0,
                         'message'      => 'foo',
                         'priority'     => 42,
                         'priorityName' => 'bar');
-        
+
         $f = new Zend_Log_Formatter_Simple();
         $line = $f->format($fields);
 
@@ -62,7 +62,7 @@
         $this->assertContains($fields['priorityName'], $line);
         $this->assertContains((string)$fields['priority'], $line);
     }
-    
+
     function testComplexValues()
     {
         $fields = array('timestamp'    => 0,
@@ -70,19 +70,19 @@
                         'priorityName' => 'bar');
 
         $f = new Zend_Log_Formatter_Simple();
-        
+
         $fields['message'] = 'Foo';
         $line = $f->format($fields);
         $this->assertContains($fields['message'], $line);
-        
+
         $fields['message'] = 10;
         $line = $f->format($fields);
         $this->assertContains($fields['message'], $line);
-        
+
         $fields['message'] = 10.5;
         $line = $f->format($fields);
         $this->assertContains($fields['message'], $line);
-        
+
         $fields['message'] = true;
         $line = $f->format($fields);
         $this->assertContains('1', $line);
@@ -91,28 +91,50 @@
         $line = $f->format($fields);
         $this->assertContains('Resource id ', $line);
         fclose($fields['message']);
-        
+
         $fields['message'] = range(1,10);
         $line = $f->format($fields);
         $this->assertContains('array', $line);
-        
+
         $fields['message'] = new Zend_Log_Formatter_SimpleTest_TestObject1();
         $line = $f->format($fields);
         $this->assertContains($fields['message']->__toString(), $line);
-        
+
         $fields['message'] = new Zend_Log_Formatter_SimpleTest_TestObject2();
         $line = $f->format($fields);
         $this->assertContains('object', $line);
     }
+
+    public function testCallbacks()
+    {
+        $fields = array('timestamp'    => 0,
+                        'message'      => 'foo',
+                        'priority'     => 42,
+                        'priorityName' => 'bar',
+                        'custom'       => array('Zend_Log_Formatter_SimpleTest_TestObject3', 'getValue'));
+
+        $format = '%timestamp% %priorityName% (%priority%): %message% %custom%' . PHP_EOL;
+        $f = new Zend_Log_Formatter_Simple($format);
+        $line = $f->format($fields);
+        $this->assertRegExp('/Method value$/', $line);
+    }
 }
 
 class Zend_Log_Formatter_SimpleTest_TestObject1 {
-  
+
     public function __toString()
     {
-        return 'Hello World';      
+        return 'Hello World';
     }
 }
 
 class Zend_Log_Formatter_SimpleTest_TestObject2 {
 }
+
+class Zend_Log_Formatter_SimpleTest_TestObject3
+{
+    public static function getValue()
+    {
+        return 'Method value';
+    }
+}
Index: library/Zend/Log/Formatter/Simple.php
===================================================================
--- library/Zend/Log/Formatter/Simple.php	(revision 18428)
+++ library/Zend/Log/Formatter/Simple.php	(working copy)
@@ -70,6 +70,12 @@
     {
         $output = $this->_format;
         foreach ($event as $name => $value) {
+        	// String callbacks are not handled to avoid side-effects
+        	if (is_array($value) && is_callable($value)) {
+        		$value = call_user_func($value);
+        	} elseif (is_a($value, 'Closure')) {
+        		$value = $value();
+        	}
 
             if ((is_object($value) && !method_exists($value,'__toString'))
                 || is_array($value)) {
Index: library/Zend/Log/Formatter/Xml.php
===================================================================
--- library/Zend/Log/Formatter/Xml.php	(revision 18428)
+++ library/Zend/Log/Formatter/Xml.php	(working copy)
@@ -75,6 +75,13 @@
         $elt = $dom->appendChild(new DOMElement($this->_rootElement));
 
         foreach ($dataToInsert as $key => $value) {
+            // String callbacks are not handled to avoid side-effects
+            if (is_array($value) && is_callable($value)) {
+                $value = call_user_func($value);
+            } elseif (is_a($value, 'Closure')) {
+                $value = $value();
+            }
+
             if($key == "message") {
                 $value = htmlspecialchars($value);
             }