Here you go. The 103MB download isn’t 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 10.5
• iTunes 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.
For information on the security content of this update, please visit: support.apple.com/kb/HT1222
iTunes 10.5 requirements
• Mac computer with an Intel, PowerPC G5 or G4 processor and 512MB of RAM
• To play Standard Definition video from the iTunes Store, a 1.0GHz PowerPC G4 or faster processor and 512MB of RAM is required.
• To play HD video, an iTunes LP, or iTunes Extras, a 2.0GHz Intel Core 2 Duo or faster processor and 1GB of RAM is required.
• Screen resolution of 1024×768 or greater; 1280×800 or greater is required to play an iTunes LP or iTunes Extras
• Broadband Internet connection to use the iTunes Store
• Apple combo drive or SuperDrive to create audio, MP3, or back-up CDs; some non-Apple CD-RW recorders may also work
• Apple SuperDrive to back up your library to DVDs; some non-Apple DVD-RW drives may also work
• Mac OS X version 10.5 or later
• Safari 4.0.3 or later
• 200MB of available disk space
For more information
Using iTunes: Open iTunes and choose Help > iTunes Help. Type a question in the search field, or click “What is iTunes?” or a help topic.
Latest product news: See the iTunes website at www.itunes.com. For the latest information about iPod, see www.apple.com/ipod. For the latest information about iPhone, see www.apple.com/iphone. For the latest information about iPad, see www.apple.com/ipad. For the latest information about Apple TV, see www.apple.com/appletv.
iTunes Store billing questions: See www.apple.com/support/itunes/store.
Using your iPod with iTunes: Open iTunes and choose Help > iTunes Help or iPod User Guide, or see the documentation that came with your iPod.
Using your iPhone with iTunes: Open iTunes and choose Help > iTunes Help or iPhone User Guide, or see the documentation that came with your iPhone.
Using your iPad with iTunes: Open iTunes and choose Help > iTunes Help or iPad User Guide, or see the documentation that came with your iPad.
Using your Apple TV with iTunes: Open iTunes and choose Help > iTunes Help or Apple TV Setup Guide, see the documentation that came with your Apple TV, or see www.apple.com/support/appletv.
Additional help with iTunes, iPod, iPhone, iPad, or Apple TV: Open iTunes and choose Help > iTunes Help or Apple Service and Support.