It's definately the 11th in the UK, I have already seen a couple of adverts for it. (which announce the 11th April at the end)
Perhaps Amazon.de just isn't expecting any copies for some reason. (or they made a mistake)
I think some Game stores are holding some kind of Mari Kart tournament in the store, you get a hat for turning up and if you win I presume you get the game free or something.