From the release notes, iTunes 10.5 adds the following features:
- 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.
The latest iTunes update was also supposed to add support for iTunes Match, a cloud-based music service expected to launch later this year at $25/year, but appears to have been pulled at the last minute, according to MacRumors.