Are there any statistics regarding the penetration of iOS 4.0 compared against an earlier version? Are there compatibility issues [i.e. if i built against an o

Are there any statistics regarding the penetration of iOS 4.0 compared against an earlier version? Are there compatibility issues [i.e. if i built against an older version, would that run on the latest version of iOS]? If not, why would people have to rebuild? I'm specifically looking at this statistic: "186 apps were built with pre-4.0 sdks"