Neil Hughes for appleinsider.com:

Developers were asked to indicate, on a scale of one to 10, how difficult it might be for them to change their applications for the new screen sizes. On average, developers at WWDC said the difficulty would be just a 3.4 out of 10, suggesting they don’t see it as a major issue.

And as Jim Dalrymple puts it:

I’m not a developer, so I don’t know how difficult this would be, but if they’re not worried, neither am I.

Ditto