If an XML <summary> of a class contains something like <see cref="System.Double"/> then that appears literally in the tooltip on the class box in the Designer. In other locations in Visual Studio 2008, it is properly processed into System.Double before display in a tooltip, like in a code text editor.
I don't think Class Designer will interpret the summary...Thank you for the feature request. I'll put it into the feature request list for the feature team to triage.