There's been a number of requests for enabling view model first support as well as the already implemented view first approach for the WinRT version. It was also one of the last features we wanted to get integrated before rolling out a nuget version as this
introduces some breaking changes.
All the breaking changes as of commit #e23acecba6bb are isolated to App.xaml.cs.
As well as introducing support for both composition approaches it's also been reworked to do less work in CaliburnApplication to better enable you to add features such as Search, Launch arguments etc.
I've posted a full discussion of the changes at View or View Model first in Caliburn Micro WinRT.
The WinRT sample also has this article included, and as an example of using these methods shows implementing the Search and Share Target contracts.