by mike

Parentheses may also be nested (with one set (such as this) inside another set). This is not commonly used in formal writing [though sometimes other brackets (especially parentheses) will be used for one or more inner set of parentheses (in other words, secondary {or even tertiary} phrases can be found within the main sentence)].[citation needed]

This lovely wikipedia article no longer exists. I came across it on the xkcd blag: http://blog.xkcd.com/2011/03/08/parentheses/, where prophetically he comments that posting a blog entry on a wikipedia article usually causes it to be rewritten.