Entry Level

• Strong .NET 2.0 Background & willing to learn!
• Explain dependency properties?
• What's a style?
• What's a template?
• Binding
• Differences between base classes: Visual, UIElement, FrameworkElement, Control
• Visual vs Logical tree?
• Property Change Notification (INotifyPropertyChange and ObservableCollection)
• ResourceDictionary - Added by a7an
• UserControls - Added by a7an
• difference between bubble and tunnel routing strategies - added by Carlo
• XAML

Mid-level

• Routed Events & Commands
• Converters - Added by Artur Carvalho
• Explain WPF's 2-pass layout engine?
• How to implement a panel?
• Interoperability (WPF/WinForms)
• Blend/Cider - Added by a7an
• animations and storyboarding
• ClickOnce Deployment
• Skinning/Themeing
• Custom Controls
• How can worker threads update the UI?
• DataTemplate vs HierarchicalDataTemplate
• ItemsControl vs ItemsPresenter vs ContentControl vs ContentPresenter
• Different types of Triggers
• How to do Animations through StoryBoards

Advanced

• Example of attached behavior?
• What is PRISM,CAL & CAG?
• How can worker threads update the UI?
• WPF 3D - Added by a7an
• Differences between Silverlight 2 and WPF
• MVVM/MVP - Added by a7an
• WPF Performance tuning
• Pixel Shaders
• Purpose of Freezables

Rest here : Interview questions: WPF Developer

Off the top of my head:

• What is a Template, and why are they used.
• How does WPF/SL manage resources in XAML
• What is the "x" xmlns
• What are markup extensions
• What is data binding, talk about it a little, tell me why it's awesome

• What features are only available when using Silverlight Out of Browser?
• Explain how the Print functionality works in Silverlight 4? and any issues you can think regarding it?

• What are the different ways Silverlight can access Services?

  This site has some useful questions for basic Silverlight Knowledge - http://www.codeproject.com/KB/silverlight/QuestionsOnSilverlight.aspx