This is a core issue in antitrust. One factor is the SSNIP test. See http://en.wikipedia.org/wiki/SSNIP

"The relevant market consists of a 'catalogue' of goods and/or services which are considered substitutes by the customer. Such a catalogue is considered 'worth monopolising' if should only one single supplier provided it, that supplier could profitably increase its price without its customers turning away and choosing other goods and services from other suppliers."

So the question is this: if Apple were to increase its share of revenue by 5% to 10%, would developers move to Android, BlackBerry, or WP7? Absolutely. Even at the current level, there is plenty of cross-over between Android and iOS applications. In antitrust terms, the AppStore is not a market.

Sorry, I deleted my previous response as I wasn't sure it was adding to the discussion - I've added the original text below.

Thanks for the clarification - but how can you be sure that developers would move platforms?

--

I'd like to know what your definition of a market and marketplace is.

I'd define a market as a place (or economic 'space') where people sell a related class of products or services.

I'm not defining an iOS or iOS-based device as a market. I'm referring to the market for iOS applications (i.e. the AppStore).

Why is the AppStore not a marketplace?