I don't really see this as a good thing. Apple has built its model based on the fact that they're a closed system. It's how they drive their value proposition. They have a closed system and they're good at what they do, so that all of their products "just work" together, no exceptions. Third party devs could totally mess this up.
I've been assured by Chew (over on ATGAR...ha) that this is probably just giving third parties access to certain APIs vs actually opening their system up, but still...that seems to leave a lot of room open for third party devs to mess things up (intentionally or unintentionally) for the end user and tarnish Apple's brand in the long run.
To be fair: third party devs are where most of the good stuff comes from. The difference is Apple tends to watch what devs do and then duplicate it on their own time in their own environment. I'm not saying this is a bad idea in theory, more that it's just a bad idea for Apple and its brand specifically.