Say what you will about platforms, but in 2009, Apple's App Store absolutely pwnd the paid mobile app space—selling 99.4% of the $4.2 billion market single-handedly, according to ars technica.
All this data is based upon research by Gartner, who claims that, should sale trends continue, Apple could retain 2/3 of the paid mobile app market into 2010 (amidst growing competition from Android, Palm, RIM, etc). Of course, if Apple releases an app-wielding tablet, market share could error greatly in their favor again.
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(Note: there's some discrepancy in these numbers, as Engadget counters ars' 99.4% number crunching with a 97.5%. Whatever, either way, it's enough of the market to mean every other retailer was moot in 2009.)[Ars and Gartner]