I notice with some Joomla sites that they have no powered by Joomla sign at the bottom while others have and my one has. Does one without mean it's not a Joomla site and if someone removes that sign is it against the terms of Joomla? I noticed they advertise the sites in Joomla site showcase and Joomla site directories but can never see proof it is created by the CMS.
GPL requires that copyright notices stay in source code: there is no requirement for copyright notices to stay in the output, if that makes sense. Note, though, that templates have their own copyright and so may require leaving acknowledgment.
If I was you I would have deleted the joomla part in the code aswell. Due to the security risk. just put "//" before this sentence: <meta name="Generator" content="Joomla! - Copyright (C) 2005 - 2006 Open Source Matters. All rights reserved." />