Simply install the GetResponse Joomla! 1.5, 1.6, 1.7, 2.5, 3.0 and 3.3 Native plugin, give it the list name and your API key (provided by GetResponse), and all new Joomla! signups are then added to your GetResponse list as well.
It's lightning fast and extremely reliable - and of course, since you are doing things the "approved" way, your technical solution is 100% in compliance with GetResponse's policies.
Hooking into the core Joomla! registration process, the system works with all Joomla! native components, including Community Builder and VirtueMart.
UPDATE v5.9: updated for Joomla! 3.3