An Apple software update has deprecated iOS 4.3 for all future App Submissions.  The App Store will no longer accept apps submitted using older iOS 4.3 or older standards.  Users on older devices will have to update to the latest version of iOS in order to use any recently updated Apps.

Although statistics are not formally released by Apple on a regular basis, it is estimated that less than 10% of the market is running on iOS 4.3 and that the impact on users will be minimal.  Please see the article below for some further research on iOS distribution.