А еще короче?

def camel(s: String, i: Int = 0, state: Int = 0): String = {
  if (i >= s.length) "" else {
    val c = s.charAt(i)
    c match {
      case '_' => camel(s, i+1, (state + 1) & 2)
      case _ => (if (state == 2) c.toUpper else c) + camel(s, i+1, 1)
    }
  }
}