In a WPF application I have the following application resources defined:

    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary>
                    <local:Bootstrapper x:Key="Bootstrapper" />
                </ResourceDictionary>

                <ResourceDictionary Source="/MyNamespace;component/Styles.xaml" />
            </ResourceDictionary.MergedDictionaries>
         </ResourceDictionary>
    </Application.Resources>

however, the styles defined in Styles.xaml are not cascaded to the main window (I have tried the styles resource both before and after the bootstrapper resource).

Are resource dictionaries not cascaded by Caliburn.Micro?

I have all of mine before and it works just fine... I also have a few global converters in the same resourcedictionary tag set as the bootstrapper.

Interesting, styles in that resource dictionary seem to be cascaded, but not the merged resource dictionary from the second assembly.

how are you being the second assembly into the mix?  direct reference or discovery?

Have you tried using the dictionary full absolute Uri?

I mean something like

pack://application:,,,/ASSEMBLY_NAME;component/PATH_TO_RESOURCE

that is actually how the second assembly that holds my styles does the referencing.