Mike Ash at Rogue Amoeba has published his fairly extensive thoughts on Apple's code signing policies and plans, as well as how they relate to the iPhone SDK. He makes some solid points and elaborates on thoughts that are being bandied about elsewhere on the 'net. In his critique of some points in the iPhone SDK announcement, his concerns regarding the "banned" iPhone apps are quite valid, in my opinion. Out of the list of apps to be denied (illegal, malicious, unforeseen, privacy, porn and bandwidth hog), he picks out a couple that are of concern.
In regards to the issue of "porn", he notes that "...Apple is making moral judgements of the apps they sign." To me, it seems like Apple chose the safe option and just categorically denied materials that could sully their reputation, which I personally think was a good (if not obvious) choice. But the question arises, as it always does, about the definition of porn and obscenity... and who makes the call. Apple, as gatekeeper, gets to make those decisions for all of us. I can see some torrid debates arising in the future.
Also of particular (and potentially more controversial) concern is the category "unforeseen," which provides a fairly broad scope for Apple to add to the list later. Again, it's likely a smart decision on Apple's part and a good way of sealing off loopholes without making the list read like a legal contract (see "License Agreement"), but leaves open the option for some heavy-handed control over what you can put on your iPhone.
Of course, this initial list is incomplete, with restrictions outlined in the SDK license agreement (as pointed out in Rogue Amoeba's subsequent post). If you take an interest in this debate, be sure to check out Mike's post, "Code Signing and You."