We are not aware of any issues caused by simply including a RadExpander control in a Silverlight application.
I am attaching a sample project - please take a look at it and let me know if you have problems with it.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items.