Bodhesatva, you are correct, third party companies are pretty much afraid of the Wii. They expected it to fail, hate the change in game culture it presents, and Sony gave them a much bigger (console-wide) marketshare to swim in (ie Sony First Party <<<<<<< Nintendo First Party.)
There are a few situations that can result from this.
1) New 3rd party companies step up. Many 3rd party publishers have died (Acclaim) and others have stepped in to fill the void. If the "Old Guard" is willing to ignore the money on the table, the new kids will sweep in and take over.
2) No growth. Companies can ignore the Wii and make games for the other consoles. They might break even while Nintendo prints more money. Stockholders are going to eventually ask "why aren't you making games for the Wii?" And they'd better think up a good answer. Especially if the Wii's userbase greatly dominates the others.
Basically, I don't see how 3rd party companies will be able to ignore the Wii. Consumers don't care about who published the game. Most don't care about developers, either (see Guitar Hero.) All they care about is where the good games are. Nintendo will have many high-quality games with a huge userbase, and that is where game sales will go.
I would go even farther than that. I want you to look at this thread:
If you combine the net income of all the top 15-20 major third parties over the last 6 months, the total comes to -458 million dollars.
It's really shocking to see that and then see these companies refuse to change in this environment. Compare this to Nintendo's ~1.2 billion dollars in profit, for instance. You say ignoring the Wii could have significant consequences. I'd say, if the market continues in a remotely similar fashion for even 2-3 years, we'll see profound and extensive redistribution of the market place. Nintendo could swallow the market whole.
Simple question for someone who knows more about financial trends than I do: how long can a market have one company make profoundly more profit than the rest of the industry combined, and not have that market distort dramatically around the profitabl company? How long before that company consumes the entire industry?