"Rock Band: Nickelback" was codename for the Rock Band Network

...in order to "deflect all curiosity" about the project.

Harmonix knew that they had something big on their hands with the Rock Band Network, a public track development system which may produce a huge expansion in content available for the game. So big was it, that in order to keep it under wraps, they had to give it a name that nobody would be interested in.

That's where Nickelback comes in.