graeme_p - 7:13 pm on Feb 17, 2012 (gmt 0)
The original article is better than the one linked above.
The researcher's article actually explains it properly:
Once a site sets one cookie, it is no longer blocked from setting third party cookies.
That last bit is a shortcoming of Safari. Incidentally, Google have patched this in webkit, so Apple can fix this by merging Google's code!