The Unofficial Apple Weblog reports that the new version of OS X (btw - as you can see Apple dropped the "Mac" name from its system) will not support older Macs. We shouldn't be surprised - the same situation occured with Mac OS X Lion, and in 2009 Snow Leopard made an even bigger shift (all PowerPC Macs were dropped). According to the website, OS X Mountain Lion will only work on the following Macs!

• iMac (mid 2007 or later)
• MacBook (13-inch Aluminum, 2008), (13-inch, Early 2009 or later)
• MacBook Pro (13-inch, Mid-2009 or later), (15-inch, 2.4/2.2 GHz), (17-inch, Late 2007 or later)
• MacBook Air (Late 2008 or later)
• Mac Mini (Early 2009 or later)
• Mac Pro (Early 2008 or later)
• Xserve (Early 2009)

What it means? That these Macs will not be supported (via TUAW):

• Late 2006 iMacs (iMac5,1, iMac5,2, iMac6,1)
• All plastic MacBooks that pre-date the aluminum unibody redesign (MacBook2,1, MacBook3,1, MacBook4,1)
• MacBook Pros released prior to June 2007 (MacBookPro2,1, MacBookPro2,2)
• The original MacBook Air (MacBookAir1,1)
• The Mid-2007 Mac mini (Macmini2,1)
• The original Mac Pro and its 8-core 2007 refresh (MacPro1,1, MacPro2,1)
• Late 2006 and Early 2008 Xserves (Xserve1,1, Xserve2,1)

"f you're running Lion and not sure if your Mac is supported under Mountain Lion, go to the Apple Menu and select "About this Mac." Click "More Info," then click "System Report." You'll be presented with a window showing a Hardware Overview, and your Model Identifier will be the second entry from the top. If your Model Identifier matches the list above, then regrettably OS X Lion is the end of the road for your Mac."

Source: http://www.tuaw.com