And more specifically, portable computers, but not desktops.
Really? I selected US warranty for iMac and got:
Apple One (1) Year Limited Warranty
I will admit I didn't read it 100% but I did not see a section like the one I quoted earlier about service being limited to the country of purchase.
Everything is kind of pieced together from different sources. You can read the terms of Apple's worldwide warranty but it doesn't specifically mention that iPhones or cellular iPads aren't included. However, I think that's a matter of the warranty terms that are included with the phone itself.
Here's the worldwide warranty terms that mention portable devices. We all know that it doesn't include iPhones, but it's kind of hard to know that if all one does is read these terms in a vacuum.
OBTAINING WARRANTY SERVICE
Please access and review the online help resources referred to in the documentation accompanying this hardware product before requesting warranty service. If the product is still not functioning properly after making use of these resources, please contact the Apple representatives or, if applicable, an Apple Authorised Service Provider located using the information provided in the documentation. An Apple representative or Apple Authorised Service Provider will help determine whether your product requires service and, if it does, will inform you how Apple will provide it. Apple or its Apple Authorised Service Providers will provide warranty service on products that are tendered or presented for service during the Warranty Period, as permitted by law. If the product is portable, meaning that it can operate independently without a power cord, you may obtain warranty service worldwide. However, service will be limited to the options available in the country where service is requested. If the product is not portable, warranty service may be restricted to the country where the product is purchased. Service options, parts availability and response times will vary according to country. You may be responsible for shipping and handling charges if the product cannot be serviced in the country it is in. In accordance with applicable law, Apple may require that you furnish proof of purchase details and/or comply with registration requirements before receiving warranty service. Please refer to the accompanying documentation for more details on this and other matters on obtaining warranty service.