Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
iPhone 5 icon found in Photo Stream Beta ? –
Apples most recent Photo Stream Beta application seems to hold some icons which match up the rumours of which suggest the iPhone 5 or iPhone 4s will have a bigger screen and also an elongated home button. This maybe just a coincidence but maybe Apple has let this one slip through the net.