It is b27 not b26
Java Manual update is not using modified mapped XML file.

How to reproduce

1.) Install 6u10 b27 pit

2.) Set the UpdateServletURL to valid XML file. My mapped XML file
<?xml version="1.0" encoding="ISO-8859-1" standalone="yes" ?>
  <java-update-map version="1.0">
  <mapping>
   <version>1.6.0_11</version>
<url>http://j2se.east.sun.com/arc/1.6.0_10/pit/2008-06-19.12_25/windows-i586/iftw/1.6.0_11-b327.xml&lt;/url>
  </mapping>
  </java-update-map>

4.) Launch JCP and perform Manual Update via clicking "Update Now" button in update tab.
Result - It will say you have updated version.

5.)So far so good. Now, modify the mapped XML file so that it points to a correct version and update XML file
<?xml version="1.0" encoding="ISO-8859-1" standalone="yes" ?>
  <java-update-map version="1.0">
  <mapping>
   <version>1.6.0_10</version>
<url>http://j2se.east.sun.com/arc/1.6.0_10/pit/2008-06-19.12_25/windows-i586/iftw/1.6.0_11-b327.xml&lt;/url>
  </mapping>
  </java-update-map>
5.) Wait for some time or do reboot/logoff and repeate the step no#4. Result is same.

6.) Now load the mapped XMl file in IE and repeate step#4. It will show the update notification dialog.

Now try the same mapped XML file to do the update on another systesm.

1.) Install 6u10 b27 pit

2.) Set the UpdateServletURL to valid XML file. I have used same correct XMl file (as mentioned in step 5) My mapped XML file
 <?xml version="1.0" encoding="ISO-8859-1" standalone="yes" ?>
  <java-update-map version="1.0">
  <mapping>
   <version>1.6.0_10</version>
<url>http://j2se.east.sun.com/arc/1.6.0_10/pit/2008-06-19.12_25/windows-i586/iftw/1.6.0_11-b327.xml&lt;/url>
  </mapping>
  </java-update-map>

4.) Launch JCP and perform Manual Update via clicking "Update Now" button in update tab.
Result - It will disply the update notification dialog. saying that update is available.

Manual update should notify the user after mapped XML file points to a correct verison and location.
Please let me know, if you want the machines to reproduce this issue.