This is a step into the wrong direction. I hope, that more people will change to Adroid and other more open environments as a clear answer to this restrictive policy. Maybe even Windows on mobile phones might become an alternative for some game engines in the future.

To develop a game in C# in Unity is a great advantage. You can use the same code for different platforms. It simply would be a step back to write iPhone games in Objective C instead.

In the end the most interesting question is:
Will it work? Will Apple win developers with this strategy and get rid of cross-platform to harm Windows, Android, Symbian whatever with Apple-exclusive applications?
Or will it work the other way around? Will Apple lose developers? We have to keep in mind that there is no good development tool from Apple, there is no game engine, everyone has to write its own now.

This is a big step back into the middle age of software development.