MacRumors posted a story that Fortune has rounded up all of the current thoughts on why the Mac Pro is the only viable candidate

-A 100 million factory, Dan Luria, a labor economist at Michigan Manufacturing Technology Center told Bloomberg, employs about 200 people and produces about 1 million units per year
-Last year, Apple sold 18 million Macs, including 13.5 million MacBooks and 4.6 million desktops (iMacs, minis and Mac Pros). Only the Mac Pro and mini sell fewer than 1 million per year, and the Mac Pro is considerably heavier and more expensive to ship
-Apple, through a spokesperson and through Tim Cook himself, has already indicated that it will be producing a new Mac Pro in 2013

  Speculation Targets Mac Pro as First "Made in USA" Mac Product