Hi, I have the same problem using Arduino MEGA 2650.
The first configuration I tried was:
Software serial to pin 10,11 (using wire to connect the right pins from “standard shield” 7,8)
Set The JUMPER to Software serial.
The Serial Bridge software proposed in the FAQ section of the shield wiki.
This actually is not working.
So I have solved using Hardware serial, with the same wires trick (routing to arduino hardware serial port 3), bending the shield pins going to the Serial Port 0 of Arduino Mega (Serial0).
And it works!
So I think that the flaw is in the code of the serial bridge or in the softwareserial library. (for some reason the buffer lost receiving data when the buffer is larger then 64 chars, I tried also to set 255 chars in the buffer size but nothing)
Your biggest problem is that you haven’t another hardware port on Duemilanove. So my answer not solve your problem (but I hope this help you to identify the problem)