We have an American release date... I think!
May 1st.
This is not from Nintendo, but GS and IGN says it. http://www.gamespot.com/wii/driving/mariokart/similar.html?mode=versions&om_act=convert&om_clk=gssummary&tag=summary;versions
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS