updated 09:21 am EDT, Wed July 25, 2012
Upgrade should be available worldwide
As promised yesterday, the OS X Mountain Lion upgrade is now available through the Mac App Store. The download is 4.05GB, and costs $20 US. It should be available in most countries, although a MacNN attempt at downloading from the Australian store generated an error message.
The software incorporates a number of apps previously exclusive to iOS, including Notes, Reminders, Game Center, and the Notification Center. It also more deeply integrates iCloud, and introduces Gatekeeper, a new security system designed to prevent malware from being installed. In all, Apple claims that over 200 new features have been added.