App Store for smartphones is more about lock-in than app discovery. If a consumer has too much invested in iOS apps and games for example, his/her next phone upgrade will most likely be still iPhone.
Apps cost so little compared to a phone + 2 year contract that I doubt this is the reason. You are certainly locked in if iOS has an app that $foo doesn't, but you are not locked in by having to pay $1 to re-buy it for the other platform.
People look at the phone + contract costs as fixed: "I'm going to have a contract with somebody" so if they have an investment in iPhone apps that they like they are going to be more likely to stay with iPhone/ATT then move to Android. Even if it only amounts to a few tens of dollars, there's also simple inertia at work here.