What software is used to design and simulate Mac computers, iPod and iPhone circuits?
"What circuit design and simulation software runs on a Mac?"

The list of software tools for electronics design includes a bunch of tools that run on the Macintosh.
Some of those tools, such as McCAD, were originally designed to run on a Macintosh.
After Mac OS switched to a Unix base (starting with Mac OS X), quite a few tools originally designed to run on Unix were ported to the Mac.

"What software is used to design and simulate mac computer, ipod and iphone circuits?"

I do not know, but this reminds me of a funny story.
One story (perhaps apocryphal) as repeated by Gordon Bell in 1997:


  "Seymour Cray ... When asked what kind of CAD tools he
  used for the CRAY1 he said that he
  liked #3 pencils with quadrille pads.
  He recommended using the back sides of
  the pages so that the lines were not
  so dominant.
  
  When he was told that Apple had just
  bought a Cray to help design the next
  Mac, Seymour commented that he had
  just bought a Mac to design the next
  Cray."


According to the Cray Supercomputer FAQ, Apple's Cray did a lot of work in the design of the metal molds used to produce the outer case of Macintosh computers -- the Cray simulated the 3D flow of plastic and rendered it as Quicktime movies.
Although I hear that wasn't the original reason Apple bought the Cray.