Safari and cross-domain cookies

Here’s one strange issue. I’ve got an IFRAME set up in a HTML page. The HTML page is located in one domain, and the source (src) of the IFRAME is located in a second domain. This works fine in Firefox, however I ran into problems with several IE versions. Upon closer examination I noticed the wonderful world of P3P: you need to include special P3P headers in the original HTTP request, in order to make this work. The cross-domain use of cookies is usually prohibited unless you include special headers. After adding a little line in my PHP code all was fine…

header("P3P:CP='IDC DSP COR ADM DEVi TAIi PSA PSD IVAi IVDi CONi HIS OUR IND CNT'");
header("Location:index.php");
exit();

That was all the header info IE needed. I’m not quite sure what this line meant, but hey, it worked. And in Firefox as well. In Safari however, there is no way to get cross-domain cookies to work. Not with the line above and not with any other Javascript-related workaround. But I am sure there must be a way to work around this. Or through this… Anyone?