I’ve been come really enamoured with the possibilities of Silverlight / WPF Behaviors, broadly they can be divided into three types, Triggers, Actions and Behaviors. The ability for non-programmers to compose behaviour and interactivity in Blend is very interesting.
A question I see a lot on sites like Stack Overflow and the Silverlight forums is around sharing or reusing types in WCF Service References, either between multiple services or between the client and server.
I posted a while ago about when first playing with Silverlight 3 Behaviors that you can't data bind to them. The reason being is that the base classes for Behaviors, Triggers and Actions are DependencyObject which Silverlight (but not WPF apparently) doesn't support data binding.
A month or two ago I wrote about a bug in Silverlight 2 regarding to ComboBox and changing the ItemSource (Gotcha when databinding a ComboBox in Silverlight).
The navigation framework in Silverlight 3 is a welcome addition as it reduces a lot of plumbing code required to start building multi-page Silverlight applications. One thing that multi-page Silverlight applications need is analytics.