Yes indeed, I'm super serial right now. Or is that supposed to be 'super cereal'?
I carefully followed the instructions for disabling login via the Pi's serial port. Then I disabled the Pi's boot messages (out to the serial port), shut down the system, set the jumpers to connect the Pi's serial transmit line to the Gertduino's serial receive line (and vice-versa), powered up the system (it booted up just fine, yay!), installed the minicom terminal emulator, installed the scrot screen capture utility, brought up the LXDE graphical environment, started minicom at 9600 bps over port /dev/ttyAMA0, started the arduino IDE, loaded, compiled, then uploaded the simple sample serial program Serial_startup.ino to the Gertduino, and this was the result:
(I also tested keying into minicom and watching it echoed back, so both directions seem to be working.)
To set up the jumpers to let the Pi talk to the Gertduino, you need to hook them up in a criss-cross pattern. I used a hand wire-wrap tool and two short, pre-stripped lengths of #30 Kynar wire to make the connections. Here's a blurry photo (sorry) showing the jumpers:
I hereby declare phase 1 of my phased approach to getting the two boards talking a success. Phase 2 will involve a Python program talking to an Arduino program.
But before I go there, here's a really artistic photo showing the Gertduino's blue LEDs brightly lit up to celebrate the achievement: