[*]Market driver is there are "enough: people who are willing to buy your stuff to make it worth your effort
There will be an app store attached to the n900 and i assume future instances of maemo devices... Ovi as a service and distribution platform will be pervasive across all Nokia products. You will have the ability to not use it, as a developer or a user. That will be the decision you have to make. Since this phone is being picked up by carriers and is a powerful (normal sized pocket) device that is cool, it will get picked up by a majority of people who prefer the app store route.