Whilst I do think that getting broadband if possible is a good idea, I don't think anyone should be alienated because they don't have/cannot get broadband.
I've seen a video of the supposed Mario Kart DS playing online over 56K. As long as you don't use too much bandwidth with everything else, I think it should be fine. Heck, if they can fit 8 Mii's into a 4kb space, surely they'd be able to send data along 56K line?!
You can see footage here