Single Status Update
I have a UK 3DS as well, and I've had no problems using the eShop on that via the UK eShop. With the 3DS it is console specific, so a 3DS bought in Europe will only work on the European 3DS eShop. I should imagine that it'll be the same for the eShop on the WiiU, and if it is I have a UK VISA card so no problems spending on it. So, as you have an American console it'll automatically put you through to the American eShop.
The only downside I can see, may or may not happen, is the Premium Nintendo Network Discount Deal. I read in these forums that someone in our situation, playing a console in a region different from purchase, could get the eShop but not the 10% discount vouchers. We'll both see soon enough though.