Today, the japanese website of Super Mario Galaxy 2 updated and we can see Luigi is in! Maybe the 250 stars promised by Miyamoto are splitted between Mario and Luigi.

You can see also the setting of World Maps.

They work like this:

You travel between galaxies. You enter the Galaxy: You got numerous planet and unique planetoid systems. Each system can feature at least 3 to 6 stars. At the end, you got a boss stage. You can see Malletoid as the final boss of a world.

http://www.nintendo.co.jp/wii/sb4j/adventure/index.html

I'm less than surprised.

I wish they'd make Luigi his own character with a more unique playstyle and have him be switchable with Mario on the fly. But I'm sure it'll be just like the last game where he's just a tag-along and you can play as him at the end. And he only has minor alterations to Mario's character such as sliding and a slightly higher/farther jump.

Why can't they make a game staring Luigi like he is in Smash Bros....  Probably because he'd be too overpowered.

I love the updates for this game. Luigi is in the game, mario has new power ups! Yoshi! These have to be the most unsurprising reveals in the history of the universe lol.

but then, everyone loved luigi in galaxy, so I don't see why hey would have removed him, it's
pretty obious he'll be playable, but knowing miyamoto there will be bigger changes

Yay Wuigi!!!!! So we have Yoshi Wuigi and the Bratz girl Rosalina

seriously the only way you could have a surprising reveal for Galaxy 2 would be if they found the 3rd Mario Brother in outer space

Introducing... ANTONIO the lost Mario Bros. and of course his evil counterpart, WANTONIO

"sigh" who cares i want a 2 player mario game where you can use Luigi as a 2 player like the old days