I know this is a REALLY old thread to be posting to. But this is one of the first google search results for "silverlight radcombobox scroll to top" So i thought i'd post for anyone else having this problem.
There is a workaround for resetting the scrollbar position in a RadComboBox. You just need to get the ScrollViewer from the template and scroll it to the top.
If you're extending the RadComboBox, here's the code for it:
var scrollViewer = GetTemplateChild(
If you're not extending the RadComboBox, you should be able to modify the code to fit your needs easily