I made the map tiles, but Tom did the UI and compositing, so I'm not sure how the layers are combined. There are more iPhone tweets than any other source, so they will tend to be the most visible if they are given equal priority.
As a side note, I do find the map very interesting - it's just the article's conclusion that I'm not convinced of.
I agree that it is too big a leap to say that phone choice always indicates wealth, but I do think there is a connection.
The point of the map isn't to show income disparity, it's to show the smartphone brand of tweeters. The fact that it incidentally does show income disparity is what's interesting.
The correct approach would be to decide a winner, or do some blending for every pixel on the map.