RadAjaxPanel is a "lookless" control and is not used for any visualization. If you need rounded corners, use additional HTML elements inside or outside the RadAjaxPanel.
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