A while back I wrote a post about using the Mediator pattern, to allow ViewModels to communicate with each other easily and simply.
Now lately the wpf disciples have been talking about this pattern a lot. Josh Smith has written a better version that is more memory safe than my orginal version. You can read about Joshs version at his blog post. The wpf disciples have also been discussing the possibilities of released a best practice set of files when working with WPF/MVVM/Mediator, which will use bit of WPF knowledge from the whole group. This will be released over at codeplex. Watch out for that one