I think it's fair to say that this is the BIGGEST announcement of third party support for the Wii to date. This is a coup, and I am just as excited as Ian as to what this could do for the Wii's reputation amongst third party companies in Japan.
However, I also think that to see this game in exclusion to other announcements is exhibiting tunnel vision. Like others have pointed out, Monster Hunter Tri was a very large coup for Nintendo as well, perhaps the biggest such victory before this DQX announcement. No matter if the series is alien to you, the game has shown a market significance amongst core gamers (it enabled the PSP to seize the top sales spots in Japan, and to gain a second wind against the DS that it is still benefitting from) and has probably done a lot to help the Wii's reputation amongst traditional consumers and game makers. Pap64 brought up Halo: If Halo was announced for the Wii, would that be blase? No matter what you thought of the game as a player, you have to acknowledge the effect it would have on others.
The same goes for many previous game announcements for the Wii, including not just Monster Hunter, but the Tales of RPGs, Dragon Quest: Swords, Tenchu IV(now heading to PSP, but I'm not about to look a gift horse in the mouth), Fatal Frame IV, No More Heroes, Bikini Zombie Slayers R, and possibly many more. These were the precursors to the DQX announcement, and may even have made DQX Wii possible.
To see DQX Wii as a singular event without acknowledging the many steps that it took to get there (which may have involved games that exhibited lesser excitement) is at the very least naive, if not woefully unrealistic. It is certainly a capstone achievement, the most exciting announcement yet, and possibly even a highly effective PR victory. But it didn't all of a sudden burst onto the scene fully formed: it is only the latest result in a long process that has bore many other fruits along the way.
*ahem*
Now, let me be negative. IT'S FREAKING TWO YEARS INTO THIS GENERATION. SE takes FOREVER to make a game: WHEN DO THEY EXPECT DQX TO RELEASE? 2010? 2011? Look, I'm all excited and all... but by the time we get to play DQX the freaking XBox720 and PS4 will be out. Maybe even the Wii2. Just how much benefit will this game really give the Wii if it comes so late in the game, huh?
Geez louise. It's taken them long enough.