The our WPF combobox should do exactly what you need, but it contains a bug that prevents the user from typing in read-only mode. See the Silverlight combo (that shares the same source code):
This problem is scheduled for fixing, most probably we will release an internal build, containing the fix before the Q3 2009 release (November). It will be fixed in Q3 for sure. I cannot promise a specific date for resolution, so I would recommend using your workaround for now. We will make sure that the bug will be fixed as soon as possible.
All the best,
the Telerik team