Here are a list of the Macs that are supported in Mountain Lion:
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)
This means the following Macs which are supported under OS X Lion will not be able to run Mountain Lion (model identifiers in parentheses):
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)
Source:
http://www.tuaw.com/2012/02/16/mountain ... ac-models/