I just got this to work after many hours of coffee fueled hacking :) The mbed can now speak the Autostars kinda-i2c-but-kinda-not language, and in the pic there has asked the Autostar to report the scopes product name and firmware date.
Next I need to look into how interrupts work on the mbed, as currently as soon as the mbed leaves the scope alone to do anything else (such as writing to the LCD) it will lose sync with the clock signal the Autostar outputs and communicating won't work anymore.
Then I need to look at the commands for moving the scope and reporting its position, and eventually write the locations of some interesting targets onto the microSD card that you can see there in the top right and hook up some buttons to select one and slew the scope to the correct position :)
I'm definitely a fan of the mbed - it feels much more powerful than the arduino I was using previously, there's a lot of example code and libraries for doing things like the LCD output and SD card read/write, and it's nice to be using a proper C++ compiler. The compiler's web based and seems to fail to save at times, though that could be the fault of the dodgy net connection I'm on at the moment... Still, I've developed a game where I try to guess how many attempts it will take to save the code so it's all good fun :P
Also, it's the day before I go to the star party - so I'm not sure if this counts as a success in my planned timing. It does technically work, even if it doesn't do anything useful yet :)