The issue isn't in your SL code, it's in your sender code. When sending the message, the default implementation uses a binary message serializer, but the SL implementation uses DataContractSerializer, which depends on XML formatted messages. To fix the issue, your message send needs to do something like this:
DataContractSerializer ser = new DataContractSerializer(typeof(string));
queueClient.Send(new BrokeredMessage("test", ser));
On Service Bus, the body content is pretty much a bag of bytes-- the sender and receiver need to agree on how those things will be encoded/decoded.