Now that -webkit-background-size is implemented, (see http://bugzilla.opendarwin.org/show_bug.cgi?id=8353), we need to add it to the background shorthand. I think we are still waiting on the CSS WG for a grammar for this that will work.
Six years on, this has still not been done: see http://meyerweb.com/eric/css/tests/background-size-specificity.html for a testcase demonstrating its absence from the shorthand. Works as expected in Firefox and Opera.
*** This bug has been marked as a duplicate of bug 27577 ***