This will be an Epic Fail right from the start. Carriers will severely limit the apps available, will be slow to add new apps, and patches and updates to the apps will be non existent. Basically they will set it up and forget about it, and then wonder why nobody uses it.
App stores are best left to the companies that have an interest in seeing them succeed (the Carriers don't).
So if I'm looking for an app I can't just look in one place, I have to check 2+ stores?
How does this benefit the consumer?
It's not like Google's market has draconian policies or exorbitant fees...
I could see how manufacturers might want to tie (dumb) developers into making apps that run only on their skinned version of Android, but that wouldn't apply to carriers...