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
Apple Release iTunes 11 To Public , Download Now!
Apple has finally put everybody out of their misery and released iTunes 11 to the public you can download it now from here. iTunes 11 is one of the biggest and best update we have seen to iTunes since Apple released iTunes back in 2001. If you want to read about all of the new features in iTunes 11 be sure to check out our in depth look at the new features in iTunes 11. If you face some problems when downloading iTunes just be patient and just remember how many people are trying to do the same thing. So our solution is if your download fails be sure to lets us know your problems feel free to email us:Tips@MacReviews.com or let us know on twitter @MacReviews
UPDATE : You can not download it from the Apple website yet just use software update and it should be there waiting for you. We have also got the direct link to download iTunes 11 here