Are you sure you're dialing it right? I just dialed wrong twice in a row (*#*# not #*#*) and got this error: "Connection problem or invalid MMI code." When I dial it correctly, my HTC G1 reports Phone information, battery info and history, and usage statistics.
Take a look at this more general question about dialer codes (aka secret star codes) for reference, too: Is there an official dialer code resource? There are some good links there for lists of dialer codes and some caveats about using them without adult supervision.
