Don't let the name fool you. Rock Band won't always be about the rock. Responding to a rumour that the game will soon be receiving some country tracks, a Harmonix staffer has said:

With so many genres represented in Rock Band already, why would the addition of Country be such a huge shock? We'll have Country in RB sooner or later. Ideally we'll have all the various sub genres that people are clamoring for (hardcore, punk, goth, ska, etc) eventually. We haven't shut the door on anything.

It's not a shock, don't worry. Those who can recall the first Guitar Hero will of course remember that the best damn song in that game was, surprisingly, Stevie Ray Vaughan's Texas Flood. Which last time I checked neither rocked nor rolled. Country comming to RB2 in December [Rock Band, via 1UP]