According to this link
According to the article, there have been 50 million game purchases through the App store.
There are 13 million Iphones sold, giving an attach rate of 3 if we assume that 0 Ipod touches have been sold. Obviously, this isn't true, but my guess is that the Iphone is more popular than the touch, assume 7 million touch sales then the attach rate is 2.5
50 million games in 17 months
= 2.9 million games/month
By comparison the PSP shipped
101.7 million games by the 27th month of its life
= 3.8 million games per month
I couldn't find PSP hardware sales for 27th month, but the attach rate would be approximately 3
And that's being generous to the PSP, because that's shipped numbers of games, not sold, and because attach rates and game sales are lower in the early years
Anyway, the Iphone is not really a threat to either Ninty or Sony, as Apple is going for market expansion, but my point is that the Iphone is very much a viable gaming platform, and should become more so in the next year or two
My data comes from
http://en.wikipedia.org/wiki/IPhone
http://en.wikipedia.org/wiki/List_of_best-selling_video_games