It's been all but officially confirmed til now but Variety has a short article that features the odd spoiler about Allysa Milano's role in the game:

The Associated Press has released a story about Alyssa Milano taking over as the female lead in Ghostbusters: The Video Game.
“Obviously, fans of the movie are going to miss Sigourney Weaver,” said Milano, who was offered the part of the game's leading lady after Weaver declined to reprise her role as Dana Barrett. “I have so much respect for that character in the movie that I hope I do the position that she is in justice. I was just flattered to be asked to be part of ‘Ghostbusters’.”
In the game, players become a “Ghostbusters” recruit who assists the original team to take down ghastly foes like the ginormous Stay Puft Marshmallow Man and god of destruction Gozer. Milano said she spent one day in the studio voicing the character of Dr. Ilyssa Selwyn, the brainy guest curator of a “World of Gozer” exhibit at the Natural History Museum.
Source: Variety.

Thanks to AJ for the above image.

Alternate Press' Youtube also has a quite bit with Milano, which features scenes from both versions of the game and contains minor spoilers here and there. View it at your own risk:

Alyssa Milano Ain't Afraid of No Ghosts

Very cool. Thanks for the find!

“I get up for girls that get up!”

I like her look in the Wii version…

Maybe this is a stretch, but Ilyssa Selwyn and Ivo Shandor have the same initials, she is the curator of the Gozer exhibit and she appears at a haunted Architectural firm. Any chance Ilyssa Selwyn is really Ilyssa Shandor? (*peter)

Finally i am happy to see Bill murry being really funny in just the two good shots of him. This game is going to be great!!!!!(*peter)

JonXCTrack;137304
Maybe this is a stretch, but Ilyssa Selwyn and Ivo Shandor have the same initials, she is the curator of the Gozer exhibit and she appears at a haunted Architectural firm. Any chance Ilyssa Selwyn is really Ilyssa Shandor? (*peter)

The most recent footage seems to reveal a possible explanation for why she was at the architectual firm, that she and the Ghostbusters fled to it to escape from Stay Puft, this was the conclusion I reached following the revelation that she first bumps into them at the Sedgewick.

Granted she bumps into them at the Sedgewick, but in the demo I played at the NYCC there is a dialogue between Ray and Peter that goes something like this after you save Ilyssa from the clutches of Stay Puft:

Ray: Pete, we need you down here.
Peter: I'm a little bogged down here. These things are nasty. They're nasty, sticky and…unhealthy!
Ray: Would it change anything if I told you there was a damsel in distress?
Peter: I'll be right there!

If that dialogue goes undisturbed, I think bumping into Ilyssa at the Sedgewick is a coincidence and that she actually works in the architectural firm.

Here's the hi-def version. There's also some added footage!

http://ps3.ign.com/dor/objects/14218851/ghostbusters/videos/ghostbusters_trl_milano_032009.html

Ghostbusters Fans now has a Press Release and a Character Fact sheet.. along with SEVERAL screenshots of Ilyssa! Also uploading a new video from Atari.

http://www.gbfans.com/community/viewtopic.php?t=10720&f=54

I had gotten the impression from a previous article that she was going to play Dana, but I'm glad she's someone altogether new. The new footage looks great.

God, this hurts. Two more months, I know, but I am absolutely aching to play this game. I'm so excited.