A great and easy way to avoid the double-bounce issue and prevent getting duplicate entries etc is to use the SingleClick property of the RadButton.
<;telerik:RadButton runat=";server"; ID=";btnSend"; Text=";Send"; OnClick=";btnSend_Click"; SingleClick=";true"; SingleClickText=";Sending...";>;
<;/telerik:RadButton>;
I've no idea how I've not encountered this RadButton setting before now, saves wiring up custom OnClientClick events...