OK, so the first step is to connect up the cables, power the server on, and configure the SP.
So I do that - system and management networks, connect up the serial port, tip in, and apply power.
Silence. Not a peep. Not a sausage.
Come on, now. Talk to me!
Now I think this is OK, because the same tip session from the same host works flawlessly on every other Sun x64 server I have. But still I go through the motions - different host, different cable, various types of cable.
It's still sulking.
I give Sun Service a call. I don't think I'm doing anything wrong, so we'll see what they have to say - maybe I've got a faulty unit.
Turns out there's a problem with some units shipped with the wrong settings. If this happens to you, connect up a real keyboard and monitor, and power up the box (the real box, not just the standby power to the SP). Hit F2 to go into setup, and look through the settings. The "external serial port" should be set to SP. If it's set to system, hit F9 to restore defaults, and F10 to save (I think it's those function keys). I get back to my desk and there's the regular SP login prompt.
Thanks to James of Sun Support for tracking that one down for me.