Whilst being at the genius bar in regent street, twice of my many occasions, they turned away someones iPhone that was purchased in America and an iPad too. I don't know why the reasons where, but both in warranty.
I personally have been to the new york store and changed my charger, no issues, in fact, I would say even more polite than normal.
The keyboard is different in the US but a minor issue, considering the amount of money you are saving.
Personally, I would buy it!
Gabi.