At the Warrior Forum, an Internet marketing forum I'm member of, one of the members wanted to restrict part of her Joomla! membership site content to registered users only, specifically the intro section of an article must be "public" but the rest restricted to paying members.

So I went searching and came across the Joomihide plugin in Joomla Extentions which seems to do just that (I didn't restrict access to the rest):

JoomiHide is a Joomla Plugin that will restrict any part of an article or content to registered users only. All you have to do is just insert the "reg" or "unreg" string into your joomla text/content then the text within the "reg" tags will be showed to only the registered users and the text within the "unreg" tags will be showed to only guests