So because Nintendo releases something to the public and press it is deemed not a spoiler. Don't you think it is up to the customer to decide what is a spoiler and what isn't a spoiler?
Seriously, Maxi, the only ones that would care are the die hard fans who put so much thought and emphasis on these things. The average fan and consumers are likely to see these videos and say "Oh, that's neat!", and think nothing else of it.
Want proof of it? Nintendo posts their trailers on the Nintendo Channel (a channel designed for the new Wii audience). These trailers are the most watched videos on the channel, many of them including "spoilers". What does this say to me? It says that people don't mind being spoiled as long as they can watch the latest footage.
Plus, Nintendo knows what they are doing. In the first Galaxy, they did keep Luigi a secret because his inclusion was far more significant there than in the sequel. That was the first time Luigi was a PLAYABLE character in a 3D Mario game (while technically Mario 64 DS was the first, it was a remake). Nintendo did their best to hide this surprise from fans (the spoiler came from a guide scan). But with this trailer, Nintendo doesn't see this as being huge enough to keep secret.
Besides...it's Luigi. Luigi has been a staple of the Mario franchise for decades. Everyone knows who he is, and everyone expects him to appear in a Mario game, one way or another. The REAL surprise would be if he wasn't included in the game, or if another character not common in the 3D Mario games (like Wario, Donkey Kong etc.) made a playable appearance instead.
Stop making a big deal out of it.