Distimo has released some insightful data on the impact of being featured on the app store. The last line is the most important

Another way to attract attention is by getting featured in one of the app stores. When looking at those applications that were already ranked among the top 100 most popular applications, the average gain in the first three days after getting featured was highest in Google Android Market (+42 ranks), followed by Apple App Store for iPad (+27 ranks), and Apple App Store for iPhone (+15). Looking at the whole period these applications were featured (seven days), the rank gains were more or less equal to the three days average in both Apple stores, but were even higher in the Google Android Market (+65 ranks).

There were also significant effects after an application was featured. In the five days after the featured period ended, the gain was still +145% in the Apple App Store for iPad, +75% in the Apple App Store for iPhone, and +828% in the Google Android Market. +828% exemplifies a rank change from 8 to 1 or 100 to 12 in the Google Android Market.

Top applications benefit disproportionately from being featured because app store download volumes are exponential in distribution. The top app generates about 5 to 10x more downloads than the number 10 app and so on.

If your app is already successful on the app store, being featured will drive huge download volumes. But if you have a lesser known application, don’t expect that being featured will drive millions or even hundreds of thousands of downloads.