I would seriously consider looking into the WPAlchemy metaboxes class. It does exactly what you are after and so much more.
http://www.farinspace.com/wpalchemy-metabox/#have_fields_and_multi
This class is so great that I am frankly shocked that there is not a lot more discussion about it. *If there is a better metabox class, I’d love to hear about it, though I am very happy with WPAlchemy.