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)