I think the competition in Silverlight Controls market will be fierce. Just under a few months, developers have created a handful of UI controls for Silverlight 2.0 already. Well, some like VectorLight provides it for free with full source code (maybe for now?). The VectorLight's controls set include Calendar, Dropdown List, Fieldset, ListBox, Textbox, TextArea, Tree View, Progress Bar, and Text Label.

VectorLight Controls Demo