It's not just Mac or Safari alone -- this affects all recent browsers (FF3 and IE7 are also known to be affected) which have introduced a new setting to refuse third-party cookies, when that setting is in effect while browsing sites such as yours, whose pages are displayed inside a frameset wrapper where the visible frameset domain is different from the domain of actual pages displayed inside that frameset.
For more details, see our Service Status and Update forum post on this very subject here:
http://www.e-junkie.com/bb/topic/2716
