I actually beg to differ with those people that were so quick to say "NO!!" As of joomla version 1.5.x there are some components that allow you to do just that, most of them being commercial but there's also http://www.janguo.de/lang-en/Downloads/func-finishdown/31/ which is free at the moment. As of joomla version 1.6.x multiple sites will be integrated into joomla.
