Just that question I want to detect the retina display in my MonoTouch app. Thanks.
There are already answers for ObjectiveC but here's a C# version:
bool retina = (UIScreen.MainScreen.Scale > 1.0);
That will work for the newer iPhone and iPod Touch and I suspect (will know in less than two weeks) for the new iPad.
Jason's approach will work too and can be attractive if you need to know several hardware related features (e.g. retina + camera).