The Nintendo Wii U GamePad High Capacity Battery (tm) is available in the UK and Germany, apparently, but it's not yet announced for North America. I imagine it will end up at a comparable price (i.e. around $40) when it ends up here, and I'm looking to get a Wii U in a month or two.
If the thing isn't available here when I get a Wii U and I import one from the Amazon UK store, is there any reason to think that it wouldn't be compatible with the US GamePad? Like, would the battery voltages be different or anything? I can't imagine why they would be since it's the outlets and the chargers that matter, not the batteries. If I use Amazon's nice global shipping option, it'll cost me around $50, which if I were buying directly from Nintendo they always charge about $5 for shipping and it wouldn't make a big difference. (That is, unless they sell the thing for $30 when it comes here or something.)