"Subject: Mountain Lion Goes GM
Mike, See content from email I received today:

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GM Seed Build Available.
A pre-release version of the OS X Mountain Lion software is now available for download from the Mac App Store. Try out Dictation, AirPlay, and more! Download instructions, including the required Mac App Store redemption code, are included in this email.

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-William R."

Apple's How to Upgrade to OS X Mountain Lion has requirements and a list of "officially" supported mac models. Note that 2006/2007 Mac Pros (EFI32 models) are not listed but here's a post with some Mountain Lion Tips for (unsupported) 2006/2007 Mac Pros.
(I also saw Netkas reported that ML has now been cleaned of any 32bit kexts.)
  

• MacBook Air (mid-2012) Review Part 2 - Samsung and Toshiba (SandForce) SSDs Go Head To Head
• No use crying over Coke spilt on a MacBook Air
  (UK story on free replacement. Normally liquid damage not covered by warranty)
• How to Tame OS X Lion Annoyances
• USB 3.0 Brings a New(ish) Plug (sponsor blog)
• (Rumors) Apple to upgrade 3rd-gen iPad to solve overheating
• (Rumor/Op-Ed) iPad mini: Apple's super-slim Amazon Kindle killer
• New Apple iPhone 5 Spider concept video released
• Review of Tomb Raider: Underworld
• OnyX 2.4.7 for OS X Lion

• About the Guest network feature of AirPort Extreme (3rd Gen or later), Time Capsule (2nd Gen or later) and AirPort Express 802.11n (2nd Generation)
• iCloud: How to get long email headers for iCloud accounts
  
  (These Docs now archived/no longer updated)
• Archived - Intel-based Macs: Forcing a Universal appl to run with Rosetta
• Archived - Intel-based Mac: Microsoft Office doesn't launch (w/update_prebinding tip)
• Archived - MacBook Pro does not display printers via AppleTalk over AirPort
• Archived - Optimizing your Mac Pro for acoustical performance
• Archived - Mac Pro: Noise, pops or clicks heard when monitoring a live recording
• Archived - Xserve (Early 2008) and Mac Pro (Early 2008): AXD and AHT not showing correct number of processors or cores
• Archived - Xserve: Unexpected reboot causes
• Archived - Xserve: Use Built-In Ethernet for Remote Setup or Installation With Server Assistant
• Archived - How to reset the SMU on a Power Mac G5 (Late 2004 or Late 2005)
• Archived - iMac G5: How to Reset the SMU
• Archived - About the iMac G5 diagnostic LEDs
• Archived - eMac: How to Install or Replace Memory

• iHome iW2 AirPlay Speaker Review
• Microsoft to buy display maker Perceptive Pixel
• Valve Announces Steam Greenlight
• Mozilla scraps Thunderbird development - email client 'not a priority' anymore
• Microsoft analyzes over 1M PC failures, results shatter some myths
• Privacy risk from ads in apps rising: security firm
• Android's Biggest Threat: Adware?
• Op-Ed on Lack of security measures in Internet-connected TVs
• Mac or PC? There's a Camp For That
• 'Portal' Lego concept could become an official product
• Source Filmmaker 0.9.5.6 Released
• New HP 23in all-in-ones
• Air Def TV/Wireless HDMI Kits from IOGear, ActionTek and DVDO
• (Preview) Emotiva XPR-5 Five-Channel Reference Amplifier
• Drones, Caves, and Toilets: When Data Centers Go Rogue
• Ken Block flies through the streets of San Francisco