Shocking, of course. iTunes has been updated to support 1080p content, plus a few tweaks which appear to help iCloud customers. Check Software Update on your Mac to apply this.
Here's the list:
iTunes 10.6 adds the ability to play 1080p HD movies and TV shows from the iTunes Store.
This release also includes many improvements for iTunes Match, including:• Improved song matching
• Improved album artwork handling, downloading, and display• Addresses an issue where songs may skip when playing from iCloud
For information on the security content of this update, please visit: support.apple.com/kb/HT1222