Is it possible to install Mountain Lion on a MacMini 1,1 (Mid 2007)?
Not without modifying the installer and dealing with any issues / drivers / recompiling some of the code as needed.
The list of Macs that Apple checks before installing is on the requirements page:
As that link could easily change within a year's time, here for posterity are the models listed as supported on Mountain Lion:
- iMac (Mid 2007 or newer)
- MacBook (Late 2008 Aluminum, or Early 2009 or newer)
- MacBook Pro (Mid/Late 2007 or newer)
- Xserve (Early 2009)
- MacBook Air (Late 2008 or newer)
- Mac mini (Early 2009 or newer)
- Mac Pro (Early 2008 or newer)