The Tech Report reports a teardown of Apple's new 2012 iMac reveals that some of these new systems are assembled in the USA.
The site also points out that a product has to undergo a "substantial transformation" in order to quality for an "Assembled in the USA", just installing some RAM and a HDD doesn't cut it.
Here's the skinny from the the FTC's Bureau of Consumer Protection:
A product that includes foreign components may be called "Assembled in USA" without qualification when its principal assembly takes place in the U.S. and the assembly is substantial. For the "assembly" claim to be valid, the product’s last "substantial transformation" also should have occurred in the U.S. That’s why a "screwdriver" assembly in the U.S. of foreign components into a final product at the end of the manufacturing process doesn’t usually qualify for the "Assembled in USA" claim.
As AppleInsider points out, this might not mean Apple is transitioning manufacturing to the States. CEO Tim Cook reportedly mentioned supply constraints during Apple's quarterly earnings call in October, so perhaps Apple is merely supplementing Chinese supply with domestically built systems. Then again, perhaps something else is at play here.