[atarashi]

the original reason behind the closed architecture of the iPhone was to keep the device secure on cell networks

That's probably true for the iPhone, but it doesn't explain the closed nature of the iPod Touch. The two devices share the same core OS, but it wouldn't be difficult to let iPod Touch users download apps outside of the app store. So if its not about protecting the cellular networks, and it's not a technological barrier, what's left? I guess you could argue it's about control or about enforcing a consistent (and "safe") user experience. I'm hoping they don't apply the same thinking to the Tablet.