RadComboBox for WPF is currently in a development stage. We are actually porting the RadControls for Silverlight to WPF by keeping the practice to equalize the API of the controls under both the platforms(SL and WPF).
The official release date of RadComboBox for WPF will be Q2 2009 due at the end of June/beginning of July.
I hope that this timeframe is acceptable for you.
the Telerik team