Currently the bson serializer cannot set properties of type byte[]. I also noticed that, in one scenario, it couldn't read a byte[] property successfully either (see http://groups.google.com/group/mongodb-csharp/browse_thread/thread/d9d3079d7ee30ada for more details).
Steve Wagner mentioned that a workaround is to change the property of the domain class to type Binary (which has conversions to and from byte[]). This workaround works and is fine, so this is just a request for the serializer to support byte[] natively.