I loved a recent interview with Nintendo. Roughly quoting:
"It's not a matter of us not being able to do it, the technology is the same as for LAN. Nintendo just doesn't want players to have to spend more money on a game they already purchased."
And also the variant:
"Anyone can put online play in a game; Nintendo wants to bring something original to the table when they do it."
So you see? They're doing us a favor by not delivering the single-most requested aspect for a new Mario Kart. That way, instead of having the choice to pay them if we wanted to play online, that difficult decision is removed, and it would be so common and urbane to make their games online without a brand new gimmick to focus on solely.
Kudos to Warp Pipe Project; I personally though there'd be a lot more roadblocks in their efforts to pull this off. My only concern is if Nintendo encodes the signal sent to try to block Online Play using something like this, but even then, I guess it wouldn't matter, just as long as Warp Pipe gets the signal from one Gamecube to another, it doesn't really have to understand specifically what is being sent...