Version number is missing in back-end 2.5.0
From Joomla! Documentation
The "J2.5" namespace is a namespace scheduled to be archived. This page contains information for a Joomla! version which is no longer supported. It exists only as a historical reference, it will not be improved and its content may be incomplete and/or contain broken links.
In older versions of Joomla the version number is displayed in the footer of the back-end.
In Joomla 2.5.0 the version is shown in the System Information and is no longer coded in the admin template for security reasons. This is inconvenient and this has been discussed in the Joomla BugSquad.
The Joomla Bug Squad has developed a little admin module that shows the version in the footer position of your admin template like
Joomla! 2.5.0 Stable [ Ember ] 24-Jan-2012 14:00 GMT
and will be updated automatically with each release in your administrator back-end.
More information, see: http://forum.joomla.org/viewtopic.php?p=2727456#p2727456