My app (written using MvvmCross and with MVVM pattern in mind) needs to display popup windows where user can choose of confirm certain options. So basically it's a classic modal dialog, but since the app's view model is implemented in a portable class library, it needs to tackle modality in a generalized sense - some platforms simply don't have exact match for a modal dialog.
There are a few threads discussing dialogs in MVVM (Open dialog in WPF MVVM, WPF MVVM dialog example). Following their advices I could probably solve this by introducing DialogService and implementing it for each platform. However I will be treating then dialogs like other services - storage service, map service etc.. But a dialog is a part of the presentation concept, so I wonder if it can be treated more like a view, so instead of calling an instance of an obscure IDialogService I could navigate to it using an MVVM framework of my choice (MvvmCross in my case).
I checked MvvmCross implementation and samples but found almost no dialog-related stuff.
Within MvvmCross, the presenter is responsible for how Views/ViewModels are shown when using ShowViewModel.
This presenter is a view/UI level object - it's ultimately the UIs job to decide if a view should be shown as a page, as a control, in a tab, in a split-view, as a dialog, etc.
v3 does introduce a presentation hint that the ViewModel can help suggest how the View should be shown - but it's up to the presenter on each platform to determine how (if) to use this hint.
Alternatively, Dialogs/flyouts/etc can easily be shown using MvxMessenger messages from ViewModel to View with a little bit of code behind.
For 'modality', also consider Greg's post on 'returning results' - see http://www.gregshackles.com/2012/11/returning-results-from-view-models-in-mvvmcross/