Apple has paid 2.5 billion to developers -- and that doesn't include other ways of making money on the ecosystem (e.g. writing apps for others)

The danger parts are if you are trying to make money in the same way Apple is, or threatening them in some way.

Some examples:

- if you are going to try to make some kind of significant (30%) affiliate payment as a go-between. (iFlow)

- if you try to fill a temporary obvious gap in iOS, that nearly every user would want -- this only works if you can keep out-innovating Apple (if they even allow it). (Instapaper -- but I think he'll out-innovate)

- if somehow your app/service gives some advantage to Apple's competitors (AdMob)