The 103MB download is not immediately visible from the iTunes home screen but clicking download reveals the 10.5 download button. It is also hitting Software Update. This is the first step in Apple's upgrade to iOS 5 and iCloud. Other updates, including iOS 5 tomorrow should be hitting shortly.Full release notes below (via MacStories):What's new in iTunes 10.5iTunes in the Cloud. iTunes now stores your music and TV purchases in iCloud and makes them available on your devices anywhere, any time, at no additional cost.Automatic Downloads. Purchase music from any device or computer and automatically download a copy to your Mac and iOS devices.Download Previous Purchases. Download your past music, TV, app, and book purchases again, at no additional cost. Previous purchases may be unavailable if they are no longer on the iTunes Store.Sync with your iPhone, iPad, or iPod touch with iOS 5.Wi-Fi Syncing. Automatically sync your iPhone, iPad, or iPod touch with iTunes any time they're both on the same Wi-Fi network.