IE is notorious for its inconsistent rendering mode. In standard-compliant mode, document.documentElement.scrollLeft/Top tell the actual page scroll offsets. But in quirks mode, document.body.scrollLeft/Top tell the truth and document.documentElement.scrollLeft/Top is always 0.

Tested with IE7 Windows Server 2003 R2 SP2.

A possible fix

Index: E:/zm/jquery/jquery/src/event/event.js
===================================================================
--- E:/zm/jquery/jquery/src/event/event.js	(revision 2158)
+++ E:/zm/jquery/jquery/src/event/event.js	(working copy)
@@ -226,9 +226,9 @@
 
 		// Calculate pageX/Y if missing and clientX/Y available
 		if ( event.pageX == null && event.clientX != null ) {
-			var e = document.documentElement || document.body;
-			event.pageX = event.clientX + e.scrollLeft;
-			event.pageY = event.clientY + e.scrollTop;
+			var e = document.documentElement, b = document.body;
+			event.pageX = event.clientX + (e && e.scrollLeft || b && b.scrollLeft);
+			event.pageY = event.clientY + (e && e.scrollTop || b && b.scrollTop);
 		}
 			
 		// Add which for key events