"Mountain Lion introduces Messages, Notes, Reminders and Game Center to the Mac, as well as Notification Center, Share Sheets, Twitter integration and AirPlay® Mirroring... The developer preview of Mountain Lion also introduces Gatekeeper, a revolutionary security feature that helps keep you safe from malicious software by giving you complete control over what apps are installed on your Mac. The preview release of Mountain Lion is available to Mac Developer Program members starting today. Mac users will be able to upgrade to Mountain Lion from the Mac App Store™ in late summer 2012.

The developer preview of Mountain Lion features the all new Messages app which replaces iChat® and allows you to send unlimited messages, high-quality photos and videos directly from your Mac to another Mac or iOS device. Messages will continue to support AIM, Jabber, Yahoo! Messenger and Google Talk. Starting today Lion users can download a beta of Messages from www.apple.com, and the final version will be available with Mountain Lion... (FYI - per Apple requires 10.7.3) Mountain Lion also introduces AirPlay Mirroring, an easy way to wirelessly send a secure 720p video stream of what's on your Mac to an HDTV using Apple TV®.

Gatekeeper is a revolutionary new security feature that gives you control over which apps can be downloaded and installed on your Mac. You can choose to install apps from any source, just as you do on a Mac today, or you can use the safer default setting to install apps from the Mac App Store, along with apps from developers that have a unique Developer ID from Apple. For maximum security, you can set Gatekeeper to only allow apps from the Mac App Store to be downloaded and installed.

Hundreds of new APIs give developers access to new core technologies and enhanced features within OS X. The Game Kit APIs tap into the same services as Game Center on iOS, making it possible to create multiplayer games that work across Mac, iPhone, iPad and iPod touch. A new graphics infrastructure underpins OpenGL and OpenCL and implements GLKit, first introduced in iOS 5, to make it easier to create OpenGL apps. Using Core Animation in Cocoa apps is easier than ever, and new video APIs deliver modern 64-bit replacements for low-level QuickTime APIs. Enhanced Multi-Touch™ APIs give developers double-tap zoom support and access to the system-wide lookup gesture. Kernel ASLR improves security through enhanced mitigation against buffer overflow attacks."

The press release sidebar has links to d/l images of the new OS.

(Developer Release) Mountain Lion Supported Macs:
It may not be set in stone, but here are reportedly the supported Macs:

• Mac Pro (Early 2008 or later)
• Mac Mini (Early 2009 or later)
• iMac (Mid 2007 or later)
• MacBook (2008 13in Alum), (Early 2009 or later 13in plastic)
• MacBook Pro (13in Mid-2009 or later), (15in Early 2008 and later), (17in Late 2007 or later)
• MacBook Air (Late 2008 or later)
• Xserve (Early 2009)

As has often been the case, despite apple not officially listing support, some other models may still be able to use it, although sometimes with a few caveats. (FYI: A 2006 Mac Pro owner has posted he's been able to install Mountain Lion on his Mac Pro 1,1.)

Messages Beta Download/Troubleshooting Info:
Apple has web pages with more info on OS X Mountain Lion including a video, developer download and a Messages Beta Download for OS X Lion users.
They've also posted a support doc on Troubleshooting Messages Beta with answers to questions on setup and uninstalling the Beta/Reinstalling iChat in Lion. (They list OS X 10.7.3 as required.)
  

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