I don't know if anything changed in XE3, but in XE2 you can do the following:
uses
Web.HTTPApp,
Datasnap.DSHTTPWebBroker;
function TServerMethods1.EchoString(Value: string): string;
var
Module: TWebModule;
begin
Module := GetDataSnapWebModule;
Result := Module.Request.RemoteIP + ': ' + Value;
end;