Currently in 1.5cvs, SpecialPage.php checks the 'delete' privilege/right for
Special:Undelete. To allow a separated right for undeleting, it should check
'undelete' is currently in $wgAvailableRights in Defines.php (see bug 2498), but
would also need to be assigned to the 'sysop' group in the default
$wgGroupPermissions in DefaultSettings.php.