After rereading what I wrote, my first post didn't make it obvious that rebooting after the fresh ubuntu install I was issued the OK prompt from the OF and then I gave the boot command to boot ubuntu.
Now I don't get the OK prompt, it just hangs.
Please can you check the connection for your keyboard? If it is not inserted correctly then the firmware will fall back to console output on the serial port (if you have a null modem serial cable and a terminal at the end, set to 115200 8n1 no flow control, you can control it through that)
PS/2 ports may LOOK snug when you plug them in, but a new port and a new keyboard combined just needs that extra bit of pressure. Turn your Pegasos off, take the keyboard out, and plug it back in again just to make sure.