Online play seems to be like the classic steering wheel/gun problem. Its great in the store, but once you get it out of the box it gets boring fast. It gets old within days because no one supports it (Who would, your selling to the smallest fraction of the market) and even with new games its the same candy in a differnt wrapper.
Nintendo are smart to be extremely weary of online play. Stuff that doesnt come with the console rarely takes off, and multiplayer is the ultimate optional accessory.
Take a look at how they are handling the GBA adaptor, they are pumping it as much as they can because they know if they dont it will fade away. They realise a $20 cable wont sell, unless they make it so that on the back of 9 out of 10 of your favourite games there is a little "GBA Adaptor" logo. Then your bound to eventually get one, just for the few crappy bonuses.
They are making it so that you dont NEED the cable, but you want it. Then once everyone has the cable, they can start actually using it properly in games.
This is how they need to handle online gaming. They need to make it an option, and even when its pronounced dead still keep implementing it in games (And getting the third parties to keep pumping it aswell). Then when its got to the point where 5 out of 10 games with multiplayer also have an online multiplayer, people will start accepting it as the norm.
The main problem with that is online gaming doesnt work without a player base. Its very hard to gradually introduce it (Granted its not impossible, but still very hard). If you get online and find the games empty every time you go in, your going to stop going in. So what could Nintendo do about this? Well, they could start out by giving a % discount off the adaptor when you purchase a game that features online play. If they could manage to knock 20% off the adaptors, even if it meant increasing the price a little (People buy expensive stuff because its cheap), they would sell a lot more. More sales means more people are capable of joining the games willy-nilly.
Now to fill those empty online games. For starters a basic game package could come with the adaptors. Even just a pile of addictive oldschool games that can be played online would build enough of a base that when you get online there is something to do. Making those garenteed hits have an online mode wouldnt do any harm either.
Now, we need a way to get the people who arent fans of multiplayer to go online. How would we do that? Well, part of answer is in the aforementioned GBA adaptor. You make it so that I can use my gamecube/GBA adaptor as a GBA hooked up to someone elses gamecube and Id be bound to have a crack at online play.
This post is completely flawed, and for the most part a rant, but I can garentee that Nintendo are thinking this in-depth about online play, and asking these questions (Plus a lot more). Why? Because Nintendo knows from experience optional accessories die fast and easy, and thats all Online play is. Heck, the GBA adaptor may just be a test too see if a successful optional accessory is even possible...