Well, I had no idea on this, but Pat did:
There is a known issue with the libgpm that was included on the 12.0 installer - it gets in the way of the serial port.
If you delete libgpm, then the serial console should work; however, once libgpm has screwed things up, it might not work without a reboot. Of course, that won't help in this case
In essence, you'd have to unpack the installer, remove the gpm library, and repack it.
A better option might be to use the installer from -current - it should work with speakup just fine last I checked (of course, that might have changed too).