Apple's iPhone App store generates an estimated $2.4 billion dollars per year

"If I were to tell you that Apple’s app economy was worth more than $2.4 billion a year, you would laugh hysterically, shake your head and walk out of the room, yes? Surf on over to some other web site? But here I am telling you exactly that! According to mobile advertising startup AdMob, there are some $200 million worth of applications sold in Apple’s iPhone store every month, or about $2.4 billion a year.
Just to put that in context, Apple says about 1.5 billion apps have been downloaded from the App Store. In comparison, the Android marketplace brings in about $5 million a month or on a run rate to do $60 million in a year, AdMob says."
Source: Research by AdMob, reported in the GigaOm blog, 27th August 2009
(I must confess that I missed this bit when I reported the original AdMob research here