Nothing surprising here. I've been keeping track of him all year, and this is just another start for him. No matter how dominant he is, I expect he'll finish out the year in AA and get shut down when the AA season is over. I know they'll be tempted to give him a look in September, but given the history of their past pitching prospects' health (Cornejo, Baugh, etc), it's best to play it safe. They showed great patience just keeping him in High A ball as long as he did. He was done proving himself there long ago.
Although for 2006, I would assume he won't be held back from the big leagues if he shows any signs of being ready, which isn't far at all from where he is now. Could be a crowded rotation next year assuming we keep the current 4, plus Ledezma improves, Douglass and Baugh show they at least can hang as back of the rotation guys, and Verlander challenges. I don't include Zumaya just yet, because his control still needs work, so AAA is probably in his future for next year.
By the way, I've heard small rumors that if the Tigers find a player available via trade that they like who would be more than a rental, that Zumaya might just be available. Of course the rumors say it has to be just the right player.