Created attachment 800 [details]
Reduced XHTML file reproducing the bug.

The stylesheet for http://www.quirksmode.org/css/firstchild.html contains the rejected rule
tr.compheader:last-child th {
	border: none;
}
Note that the :first-child pseudo class appears to work fine in the same stylesheet.

A more minimalistic show case is the attached XHTML file with embedded CSS, containing the rules:
p:first-child { color: red; }
p:last-child { color: blue; }