May be you didn't downloaded the latest version of RadControls for Silverlight - Q3 2009 SP1, because as far as I know, this is the only factor that determines which internal builds should be displayed in the page I've sent. The filtering problem you mentioned should be also fixed in the latest internal build.
Since recently we are receiving many requests for autocomplete box functionality, we will consider adding a few properties that will configure the control to behave as regular TextBox with autocomplete popup.
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