Well I am sure Metroid fans will buy the game (like me) and the game will create a larger audience for the Metroid franchise. So it may sell beetween 2 and 2.5 million.

The first metroid prime got a good marketing push, and Metroid Prime sold the most of any Metroid game to date at about 2.8 million.  I don't see this new metroid game getting anywhere near that number for various reasons.  I predict in the 1-1.5 million range.

First, Metroid Fusion (so-called Metroid 4) is considered by many (myself included) to be the worst metroid game.  The same guy who was responsible for that is now in charge of this game.  We haven't seen the gameplay, and all we keep hearing about is how they want to explore Samus as a character and her motivations.  People don't buy Metroid because they want to understand Samus, they buy Metroid to shoot at space aliens.  I admit that this could be completely wrong, but look at the trailer for Other M-- cutscenes (aka, cinimatics) about Samus.  This causes me concern in the direction for the game (Metroid Fusion suffered because it tried to tell a story about Samus's backstory that most did not care for).  I am hopeful that I will be wrong and this game will be awsome, but considering that the same guy who made Fusion is behind this I am not optomistic.

I am curious, how do you know the japanese will buy it? 

I think it will sell 2 million,but, if Metroid: Other M gets really great reviews and great advertisement it has potential to sell 3-4 million.

1.2 mil, the franchise is slowing

Read the reasons given before in the thread.  Its been stated by multiple people, including me three times.

Prime 1 got a great push and lots of industry support.  Prime 2-3 got barely a whimper and still sold 1-1.5m.  Other M is going to at least double them.

The same guy responsible for Metroid Fusion (Sakamoto) was also responsible for Metroid 1, Zero Mission and Super Metroid btw.  That's why many (including myself) are putting more faith in the game's mechanics and world design (ie: what actually matters).  I'll admit, the talked about focus on character has me a little worried (though I didn't mind it in Fusion, bar Adam's handholding), and cinematics are never my favorite thing, but the promise of a 3rd person 3D Metroid by the original R&D1 team (with Team Ninja support, makers of the best looking last gen games) has me pretty excited all the same.

Anyway, I think the emphasis on cinematics may be what actually helps sell the game in Japan.  It's also not a fps which helps immensely, not developed by westerners which also (for better or worse) helps, and the previous console Metroid games sold well in Japan (1.04m M1, 700k SM).  I'm not expecting a million seller, but I'm not expecting another sub 100k performance like the Primes.

M.O.M will sell about 1.4 Million worldwide.

Metroid has never been a huge hit, and Metroid Prime was the best selling game at around 2 million. Other M seems to be wanting to focus on the character, meaning weaker sales (character development is not a selling point in games). I said 500K, but 1 million is possible. 2/2.5 million is way too high.

Somtheing else people don't seem to be factoring is generational franchise growth on Wii. I'm not sure exactly why people think Other M will automatically undersell Prime 1 when in about every other case, we've seen Nintendo's premier franchises perform better on Wii (Mario Galaxy vs Sunshine, Smash Brawl vs Melee, Twilight Princess vs The Wind Waker, Mario Kart Wii vs Double Dash, etc). I guess you could try to point out Prime 3, but it was basically entirely unpromoted and it still outsold Prime 2...

And it came out very early in the Wiis life.  Regardless, most of the people who bought Prime 3 were just fans of the Prime series and people going to places like IGN who bought into the 'hype'.  Thgat's why this game is going to be poised to sell a lot more, due to it having thw same affect as Mario Galaxy, Zelda Wii, etc.  Its going to be 'fresh', like Metroid Prime was on the GameCube.  Difference is, Metroid Prime was on the 'last place' console before, which had a much smaller userbase than the Wii.

As I said before, it's not going to sell crazy like Mario Galaxy at 8 million, but it'll do better than most Metroids.  Including Metroid Prime I'm betting.  Most likely over 2 million and I'm guessing around 2.5-3.5 million.