I'm having a problem with the cart and Safari 3.0.4 (running on Mac OS X 10.4.11). I can add one item to the cart, but after that it forgets the item if I close the cart and try to add another item. This happens on my test store as well as this other E-junkie store.
I have cookies enabled, and it works for me with Firefox. 95% of my customers have Safari, though...