As a first step in talking about custom presenters, I created N=24 - a splitview sample.
This sample shows how to create a presenter which allows the same Views and ViewModels to be used in different ways on the iPhone and iPad:
The video is at:
A slide deck for custom presenters is:
The code is on: https://github.com/slodge/NPlus1DaysOfMvvmCross/tree/master/N-24-Split
If you want to see more N+1 videos, then please see the full and fabulous index on http://mvvmcross.wordpress.com/