My sense is that apple is much more concerned with keeping the iPhones 100% trouble-free for the 99% of people who don't want to do anything more than make calls and check e-mail than with providing development freedom for the 1% of people who are excited to start hacking their phone. (Or even just to start adding new widgets).
It's disappointing for those of us who like fun, but probably a good business decision. Suits have a lot more money to spend on phones than kids and hackers, and reliability is very important to them.
With the Sidekick service you first have to prove that you are a developer, and they give you a key which unlocks the phone for development access. When you have developed a stable app, you submit it to through an approval process and then the application is made available on the Sidekick/T-Mobile app site.
If this is the case with Apple, then they could control app approval, distribution, and even pricing through iTunes. It will likely be tightly controlled.