Currently, app markets are silos. Android, iOS, Amazon, and Windows all have their own offering. However, it appears that developers are beginning to look for alternatives to the usual suspects.
CodeNgo reported a recent study from Research2Guidance (titled "Smartphone App Market 2013") suggests there is a growing distribution in mobile app stores. Though in the report it was predicted the app store model will likely remain as the primary distribution channel for mobile apps for the foreseeable future, there is evidence to suggest things are changing. Some developers are beginning to make their apps available outside the Google Play Store and Apple's App Store, and there is value in this strategy. The Chinese web services company Baidu recently purchased the Android app distributor 91 Wireless for $1.9 billion. The Google Play store is not currently available in China, so alternative distribution channels have a lot of value.
If the growing number of apps means a greater number of "app stores," it will be harder for developers to create cross-platform software. Developers already struggle to retain interest in Windows Phone, so the there is a possibility many emerging app markets will be demographic specific - with some focusing on enterprise or business over average consumers.