Hey you! Good question. I have Diddy Kong Racing that I imported from the U.S of A and I've had no troubles getting onto Wi-Fi with it. Granted it's not been released here yet so the only people I've actually played online with have been my 'friends' or people from America, Japan etc. There is an option similar to MKDS where it asks you if you want to play against people from your local region ie. Europe, or Continental players (sounds exotic, doesn't it?!)
I've known people who've imported Animal Crossing before it was released in the UK and had them regestered on my UK game card's friend's roster. That sounds a bit muddled, but basically your American game will connect to Wi-Fi no problems and you should be able to play people from the UK online.
You have permission to shoot me if this is wrong.
