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In the UK, I tether my MacBook Pro to my iPhone 5s and use the 'All-You-Can-Eat' data on Three, they allow up to 3 devices to tether to my phone.
 
I don't think there's any carrier, at least in the US, that doesn't allow for tethering with even their most basic smartphone data plans. As I type this, I have my iPhone 4S tethered to my MBP as I take the train to downtown Chicago. Not the fastest Internet access in the world, but more than serviceable.
 
I've not used it yet with the MacBook, but I can tether it to my iPhone 5 it's on an EE plan with 20GB/month data.

Barney
 
I too use my iPhone 5s as tethering is included in my tariff. I also have a Mifi that I take with me when I need to use data aboard, mainly in Europe.
 
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