What you are seeing when RadPane becomes a floating pane is actually called a ToolWindow. You are right, by design it does have a solid background and if you wish to change that you should customize the ToolWindow through its style.
You can obtain the style by dragging a dummy ToolWindow control onto the design area in Expression Blend and generating its style. Then, delete the dummy control, you do not need it anymore. Make any customizations upon the style and apply the style implicitly.
I hope this information will be helpful.
the Telerik team
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