I've been working with the new layout system Apple added in iOS6 (Auto Layout). You basically define a set of rules like: this sticks to this, this view's height is proportional to this view's height, etc. The SDK attempts to reconcile the rules you gave and lets you know if there are any ambiguities in a clear way. It's very well thought out- powerful yet easy to use. I don't think fragmentation will be a problem.

I do think however that the extra width in Landscape will now require a completely different interface. Adding 88 extra screen pixels means that simply rotating and stretching the interface has crossed the line from "looks kinda weird" to "looks terrible".