While I’m still skeptical that there’s enough demand out there for a $60 “Rock Band” game devoted exclusively to Green Day, I’m a little less so after watching a few of the folks from Harmonix Music Systems play the game on stage during Wednesday night’s “Rock Band” party on the sidelines of the Game Developers Conference in San Francisco.

The crew played through a setlist of five songs (a medley of “Brain Stew” and “Jaded,” plus “Hitchin’ a Ride,” “Wake Me Up When September Ends,” “Boulevard of Broken Dreams” and “American Idiot”), stumbling just once. (There’s something both hilarious and charming about watching a team of game industry types fail out of their own game; the song in question was “Jaded.”)

As a music nerd who wouldn’t even rate as a casual fan of Green Day (though I’ve nothing against them), I thought the Harmonix setlist did a nice job of showing off the band’s various personalities and strengths. From what I could see from my spot near the back of the Rockit Room, the character models looked detailed and in a similar vein to those from “The Beatles: Rock Band.” I also spotted another thing the game has in common with the Beatles title: vocal harmonies. In at least a few of the songs (and possibly in all of them), I could see the game made way for two vocalists. While it’s true Green Day aren’t known for their harmonies in the same way the Fab Four are, harmonies simply made “The Beatles: Rock Band” a better game. Green Day often uses vocal harmonies, so they’re going in this game. It’s good to see the feature’s being included where applicable.

Unlike the tracks on “The Beatles: Rock Band,” the songs in the Green Day game will be exportable to other “Rock Band” games, such as the recently announced “Rock Band 3.” Whether the songs will export to “Rock Band 2” or how its vocal harmonies will carry over remains to be seen. I didn’t hear an official release date at the party, but at least one Harmonix staffer made it sound like it might be sooner, as in this spring or summer. With “Rock Band 3” slated for a fall release, I wouldn’t be surprised to see this game in June or early July. After all, what’s the July Fourth holiday without an opportunity to harmonize to “American Idiot?”

Update: A few hours after this post went live, I got a news release confirming the game’s release date of June 8. It’ll cost $60 on the Xbox 360 and PlayStation 3, $50 on the Wii. A special edition will include six extra tracks, the ability to export the game’s songs for play in other “Rock Band” games and different packaging. (Presumably the standard edition folks will have to pay a small fee to export the songs.) And, in a nice surprise, the Green Day songs currently available for “Rock Band” and “Rock Band 2” will feature vocal harmonies if you play them in the new game, along with exclusive archival material and unique visuals.