The upper echelon of applications - those with over 100,000 users - is populated by only 116 apps, which is not a big deal. After all, it's normal that only a select few applications can reach a very large user base. What's problematic is the second category of applications, those with 10,000 - 100,000 active users (active user being defined by AdMob as a user that used the application at least once during May 2009), as it is populated by only 322 applications. Together, these two categories comprise 19% out of the entire number of iPhone applications. The other 81% have less than 10,000 users, and an astounding 54% of applications have less than 1,000 users.
What does this mean for app developers? It means the App Store is overpopulated with (mostly) crappy apps. There's a good chance very few people will ever see or use your application. Quality applications should find their way to their intended audience, but even very shiny gems can be lost in a big crowd. One thing is certain: if you're thinking of getting rich with your app, you might be better off buying a lottery ticket.