Everybody is assuming that the digital signatures are intended as a way of locking everything down so that nobody can do anything without Apple's active review and consent. And perhaps that's true to some extent. But I'm hopeful that it's only part of the story.
Maybe it isn't totally a case of risk prevention. Maybe there's also an aspect of blame assignment:
Rather than imposing corporate editorial review, limiting the developer's ability to create content, maybe Apple will give away digital signing utilities to everybody who asks for one. People can sign all their own apps and just drag-n-drop them into iTunes for installation. The catch would be that every digital signature contains an identifying tag with which can be used by authorities to trace any malicious code back to the individual who created it.
Perhaps Apple would institute multiple levels of digital signatures, so that the non-reviewed code is prevented from accessing some of the most critical components from a competitive standpoint, such as the microphone of Bluetooth -- if you wanted to access that stuff, you'd have to submit your code to Apple for review.
But if you didn't need access to any of the "restricted" systems, then the free digital signature would be all you'd need - and the fact that it would be uniquely traceable back to you would be enough to keep you honest and not do any intentional harm to users' security or privacy.
This seems like it would be the best deal for them to have. Make a free digital signature for the more simple apps, and a submission signature for something that requires use of sensitive features (mic, e-mail information, etc). That way when you download that Pac-Man knockoff, you won't actually be getting your personal information stolen!