Smart forms in Rails?

You mean for instance, that if your customer has checked the box
'children', a form for the names of the children appear?

I had the same work to do for my project.
I have written my own helpers which simply render partials with rjs
(page.replace, etc.), or calling :oncomplete for auto_complete fields,
etc.
You can have a look to the krjs plugin
(http://choonkeat.svnrepository.com/svn/rails-plugins/krjs/) which is
pretty useful but a little bit difficult to customize (you might want
rewrite all completly).

By the way, I've got trouble with tabulars and page.replace but the problem was
solved on this mailing list a couple of weeks ago.

Have fun,