Apple has released iTunes 10.6. The update added support for playing 1080p resolution videos from the iTunes Store.
The upgrade is designed to make life simpler with the new Apple TV, which also adds 1080p support.
iTunes 10.6 also offers improved song matching for iTunes Match users; improved album artwork handling, downloading, and display; and fixes an issue where songs may skip when playing from iCloud. You can obtain it via the Software Updates component of Mac OS X’s Systems Preferences app.