They had been trying continuously since the late 1980's.

http://en.wikipedia.org/wiki/Taligent

http://en.wikipedia.org/wiki/Copland_(operating_system)

http://en.wikipedia.org/wiki/Rhapsody_(operating_system)

You could add Newton OS to that list. It was an attempt to do what iOS achieved: a move out of the Mac into mobile devices. Apple initially planned to implement a large number of mobile form factors based on Newton.

They also fumbled with MkLinux and BeOS, IIRC.