If you are one of the developers who think ios 4.3 is not worth supporting any more but can't be sure about it at the same time, here comes the latest stats to the rescue. I saw from live stats that currently ios 4.3.x seems to have 3% percentage in active sessions.
I think it is okay to drop 4.3.x support from now on. It's only 3% and it creates a lot of trouble for the developers who want to use ios 5 or 6 features. At the best case, your code is filled with if (IOS_VERSION > 5.0f) statements, at the worst case you get crashes in ios 4.3.x.
If you don't want to lose those guys using ios 4.3.x completely, you might degrade gracefully for them. That is some features will not be supported in ios 4.3 but they will still be able use the app with basic functionality. Try not to crash if you will follow this approach. Otherwise you can get low ratings from them.
But still, even if you drop support for ios 4.3.x, don't forget to follow MVC correctly and try not to abuse UIViewController's. See my previous blog post about it.