"WPF is the only XAML framework that uses the CommandManager to automagically raise the CanExecuteChanged event on ICommands. I never liked that approach, because of the "magic" part, but this is a "feature" of WPF and of course I have to support it. No question here.
In V5, I moved to portable class library for all the newest versions of the XAML frameworks, including WPF4.5. Unfortunately, there is no CommandManager in PCL, and I have to admit that I didn't realize that at first sight. So of course now the automagical part doesn't work anymore. Again, so sorry about that."
This seems to affect V5 - V5.01, with the fix coming in V5.02. Not long after he posted the fix, which is simply a change in namespace for the RelayCommand when using it with WPF.
"Change the namespace of the RelayCommand class from GalaSoft.MvvmLight.Command to GalaSoft.MvvmLight.CommandWpf."
This does indeed fix the issue :)