- iMac (Mid 2007 or newer)
- MacBook (Late 2008 Aluminum, or Early 2009 or newer)
- MacBook Pro (Mid/Late 2007 or newer)
- MacBook Air (Late 2008 or newer)
- Mac mini (Early 2009 or newer)
- Mac Pro (Early 2008 or newer)
- Xserve (Early 2009)
Some speculation seems to indicate the reasoning for this is related to 64bit graphics drivers do not exist for the GPU’s in these earlier models, and possibly that a 32bit EFI may be the an issue.
Others are suggesting, that this is just another way that Apple tries to force users to buy new hardware if they want to get the latest software offerings.