Official multiplayer "Hunter Mode" trailer: http://www.youtube.com/watch?v=agIQTlGNx8Q
Impressions from NWR reveal that there is indeed online multiplayer.
More info from Dtoid:
In my latest demo session, Nintendo laid out a big surprise -- Luigi's Mansion: Dark Moon has a full-blown multiplayer -- and it has local, download, and online play! In multiplayer, one to four different versions of Luigi climb the "scarescraper," a tower of semi-randomly generated room sets. You can choose from among scarescrapers with 5, 10, 25, or endless numbers of floors and three difficulty levels, normal, hard, and expert.
I spent some time playing Hunter Mode, which suggests there are other multiplayer types that they haven't yet revealed. In Hunter Mode, players must clear the entire floor of ghosts within a limited time. The gameplay is simple -- explore the mansion, find the ghosts, and suck them up with your vacuum. Time is relatively short, so it works best when players spread out to uncover the ghosts and then pair up to bring them down. Players generally play cooperatively, but in Zelda: Four Swords style, players also compete for coins.
Further impressions by IGN has more still.
Every fifth level features a boss fight. After completing levels there is a also a red coin scramble that tasks your team with picking up four randomly scattered red coins throughout the floor before time runs out -- and it runs out quickly. Do so and a power up roulette will precede your next floor, awarding items like a doubly powerful Poltergust. If someone collects more red coins than the others, they have a greater chance of winning the power-up. Adding to the mild undertones of competitively beneath this wholly cooperative mode is a statistics screen between levels that shows information like who captured the mists ghosts, who was downed the most, and so on.
Video preview by GameXplain
UPDATE: Famitsu has outlined some (vague?) details about the multiplayer modes. There is apparently contention that it has an online mode of some sort, but whether if it's online multiplayer seems to be unclear at this point.
Download Play, Local Play, Internet play.
New feature: Dark Light. The shadow of things that have been disappeared by cursed spheres (?) will appear when shone with this light.
Capturing the Sphere that will jump out when you shine the light on the area will return the object.
This does seem to confirm single card local multiplayer though, so yay!
This comes from the official website: http://luigismansion.nintendo.com/
It's also worth noting that it still says "1 player" in the sidebar.
Luigi™ returns in an all new puzzle-solving spooky adventure!
Join Luigi as he stumbles and bumbles through haunted mansions, solving mind-melting puzzles and vacuuming up ghosts left and right with the help of Professor E. Gadd and an array of gadgets.
Each mansion is filled with unique puzzles, a wide range of different ghosts, and a menacing boss battle. You'll need to use your head and the Poltergust 5000, a ghost-catching vacuum cleaner, to clean up the mansions and save the day.
With so many new places to explore, all new ways to capture ghosts, and a brand new local multiplayer mode, Luigi's Mansion: Dark Moon is another star turn for Mario's heroic brother—and it's only on Nintendo 3DS.
Vacuum if old?