my original MacBook Pro shipped straight out of Shanghai. That's where the factories and assembly plants are these days. Many of the parts may come from Taiwan though. I have an old iBook G3 here, and the box says made and assembled in Taiwan, so it may just depend I suppose. But all the other Apple products that I have came from China. Most likely the iMacs are made there as well alongside the rest of the Mac family.
You know, if you go to an Apple Store, you can buy one right there and then if you don't want to add any customized options to it. Faster shipping, no? If shipping is part of the reason why you asked, then consider this: I ordered my previous MacBook Pro on a Saturday afternoon a year and a half ago or so. It came in on the following Wednesday after just about 2 days of air shipping. And that was directly from Shanghai, so I say that was pretty good. You should be able to expect a similar wait time. Good luck with your future Mac, and hope this helped you out.