No, but Apple also creates Mac OS X which is an "open" system, i.e. there are plenty of third party applications. But there is an expectation that the applications will work to the vision presented by the vendor, because it is "best" and the users expect integration and consistency.

How this is achieved is an interesting question (and arguably one for you to work out ;-)) - probably some combination of documentation and setting the bar high - but dismissing Apple's polished UIs as "well, they've got a closed system, they can do it how they like and we can't" misses the point.

However, I expect the real reason is that Nokia have had to bend over backwards to get Skype on the N800, not the other way round. Therefore Nokia won't do anything to piss them off.

Cheers,

Andrew