Specifically this bug was found in WPF, I'm not sure of its state in Silverlight. The FontWeight is permanently some shade of Bold when using Metro. This is demonstrated in the Telerik WPF demos in the ComboBox/Theming demo.
Reason for decline: Our Metro theme was renamed and now it is called Windows8. The described behavior is expected and the reason for it is the default FontFamily which is Segoe UI SemiBold. In order to achieve bold text, you need to change the FontFamily to be Segoe UI and the FontWeight to be Bold.