I had not heard the term "butt rock" prior to today, I don't think. I read about it on Fark and apparently the original concensus was that it referred to late 80's bands and such. The more common name for that (aside from glam rock, hair metal, etc.) is "cock rock".
Someone suggested that "butt rock" included bands like Creed, Nickelback, Seether, Daughtry, Finger 11, etc. The definition that was then offered, that I'm sticking with from now on, was "if it sounds like something Chad Kroeger would crap out, it's butt rock."