There is a trick to get rid of this using the gettext
filter for translations.
This basically replaces it with nothing.
add_filter('gettext', 'remove_admin_stuff', 20, 3);
function remove_admin_stuff( $translated_text, $untranslated_text, $domain ) {
$custom_field_text="You are using <span class="b">WordPress %s</span>.";
if ( is_admin() && $untranslated_text === $custom_field_text ) {
return '';
}
return $translated_text;
}
Be aware that the version also shows in the footer and several other places, if you want a good breakdown check out the source of this plugin, it seems to have covered more or less all of it, http://wordpress.org/extend/plugins/hide-wordpress-version/