Thanks for trying out the TreeListView. Please note that it is a CTP and major changes are expected for the Q2 release. It is best not to base any production code on it.
Indeed filtering may be a bit tricky to implement in hierarchical data. In WPF this can be done by applying a filter to the Items collection on the TreeView (or any ItemsControl) but in Silverlight this is not possible.
Since the Silverlight TreeView does not support built-in filtering changing the data model will be my suggestion here.
This may sound a bit unnecessary and radical - I am building a sample project that will show how this can be done more transparently. I will follow up here when it is ready.
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system and vote to affect the priority of the items.