Something I noticed was if they were made after 2004 (previous to this like my MDD was assembled in Elk Grove, CA) and they were BTO they were assembled here. My G5 was custom ordered to have a bigger hard drive, and it was assembled here. I had an iMac (iSight 1.9GHz) that was from the factory with a bigger hard drive, assembled here. My MacBook Pro and previous MacBook Air were both base models assembled in China.
Now right before (seriously like 2 weeks!) before Apple made the old Mac Pro no longer for sale, I had to customize one as a server for 27 of the slim iMacs, and this Mac Pro had 12GB of Ram, 4x1TB, and it came from China. The iMacs however, were custom ordered to have wired keyboard and mouses were assembled here (however the base models appear to now be assembled here too). And of couse the new Mac Pros are being assembled in Texas.