The only way for Nintendo to have any success online is to offer the service for free or for cheap (though it's killing Microsoft to host all those servers at their low price point). Either of those plans would move lots of software, but it wouldn't be enough to compensate for server costs. So Nintendo could go the other route and charge a fairly high fee, and watch their software sales plummet. Kids don't have the cash for online, and adults aren't keen on yet another bill to pay monthly. It's pretty much a lose-lose situation.
If Nintendo saw the long-term success of online gaming, I'm sure they'd take a loss now to hop on the bandwagon. The reality is, though, that no one knows for sure if online is "the future." Despite the constant crowing of the online revolution by gamers on message boards, these peeps make up only a fraction of the whole market. As long as Joe Donut who buys a couple of games a year doesn't give a damn (or doesn't know about) online gaming, it's a waste of money. Until casual gamers are on board, online gaming on consoles is just another minor feature that doesn't matter in the grand scheme of things, but does take a large chunk of change out of Sony's and Microsoft's pockets.
On that note...does anyone know if EA's making money off their online games? I think they've got all their sports titles online on the PS2, for free. I'm wondering if this is actually profitable for them.