Game development is a collaborative process, so it's no surprise that Harmonix isn't going into the upcoming Green Day: Rock Band alone.
The developer confirmed with us today that it has partnered with developer Demiurge Studios, design firm MK12, and others to bring the pop punk trio to life. While you may not know their names, both Demiurge and MK12 have worked with Harmonix in the past. Demiurge was responsible for bringing Rock Band DLC track packs to retail; MK12 was behind The Beatles: Rock Band's gorgeous animated cinematics.
A blog post by a Demiurge employee (spotted by superannuation) originally brought the collaboration to light, prompting cries of "Harmonix isn't developer Green Day: Rock Band" and potentially putting the quality of the product in question.
It should be noted that outsourcing development duties (particularly when it comes to expansion-style products) isn't new in the gaming industry. The well-received Rock Band: Unplugged was developed by Backbone Entertainment alongside Harmonix, and LEGO Rock Band by Traveller's Tales.
Everyone calm the hell down. We hope to have more actual information about Green Day: Rock Band soon. You know, hopefully about the game.