The Royals are in no hurry to rush him to the big leagues, especially now that Dayton Moore has taken over. Butler won't turn 21 until next April, so Moore and company should have no real sense of urgency to push him, especially considering that the team isn't in contention. I expect him to make him MLB appearance by All Star break of 07 at the earliest and opening day 08 at the latest.
I agree with rmande09 that Butler has unlimited offensive potential. He is a plus hitter for average and power and hasn't shown any real weaknesses at the plate at any level. He has a very advanced feel for hitting, here is a quote I found from a recent interview:
"BB: I don’t necessarily take after anybody. I have a certain approach which a lot of big leaguers probably have, which is that you don’t give at bats away and you don’t swing at a pitch that’s not yours unless you have to. And that keeps you from swinging at something that you have no business swinging at early in the count. If you’re looking for a certain pitch, you shouldn’t swing for another one early in the count. Late in the count, you might have to if you’re behind in the count. It’s all about just being a smart hitter."