According to the internet, Country music is coming to Rock Band and Rock Band 2 in the form of downloadable content. The source is a poster on the game's official board citing an unlinked CMT report, so that's a bit dubious. But later on in the thread, there is a response from Harmonix's "Hardware Guru" who says:
I am open to anything new in the game. For those that complain about this, let us remember everyone is not into metal. Everyone says the name of the game is Rock Band not Country Band or Rap Band ETC. Well it is also not called Metal Band or Pop Band. When the game was coming out they stated they wanted to have music in every genre.
We're hoping for a bunch of funk content soon--so fun on drums and bass! What do you think? Should Rock Band stick to the "rock," or are you open to other kinds of music for the game? Maybe German Oom-pah or Klezmer tracks?