From the introduction of the very first Ipod apple became a true game changer.  upto that point the Mac was losing ground to the pc's and Apple was still a question mark.  Once the original iPod came out, combined with the emergence of mp3s, anyone with a 500 CD magazine in their car knew things were never going to be the same. 
Next came the touch which introduced a whole new revenue stream for Apple as the apps and games quality was unmatched.  the iPad was a natural progression ...