Hi Luc Baeten,
It is not recommended to download the current controls, because they were initialy created for 1.1alpha version of the silverlight plugin and there will be many changes in the API and generaly in the source of the controls.
We will have a public beta for Q2 (which is in July.2008).
Please open a support ticket and give us more details on your project and we will see how we can help you, according to your deadline.
the Telerik team