I've been trying for about three days, and I just can't get the OSD to boot if the serial cable is plugged in.

When I leave the serial cable unplugged, the unit boots fine. But if I plug it in, it just hangs on the orange eyeball-and-ear splash screen until I give up on it. (I've waited 15-20 minutes at least.) I don't get any output from the serial device and sending it input seems to have no effect (and is not echoed back to me). I'm pretty sure both serial ports on my machine are working, because I can connect a cable (and null modem) between the two of them, fire up minicom in two xterms, and echo characters across from one port to the other.
I've tried minicom and kermit on both of my serial ports. I'm using 8/N/1 at 115200bps (though I've tried lower speeds as well to no avail).
Does anyone have any ideas? I'd really love to hack on this thing, and it's really frustrating to not be able to play with it.
Thanks!