Various suppliers of other hardware. Updated 29/3/11
I bought a gentSTEREO-proSDM as an external USB trigger. The unit comes ready-made and was specifically designed for simultaneously trigger 2 cameras to capture images pairs for 3D photography. They also produce a gentSTEREO-REPEAT distribution board for triggering more cameras. The controller works fine with 2 cameras over short cable lengths but would not work over longer distances using USB repeaters. However, I realised that the USB signal is just a on/off pulse and discovered that if I used a powered USB hub instead of their unpowered distribution board it seems to introduce enough power into the system to transmit the pulses over much greater distances. I have tested running USB signals over 25m and it works fine. Theoretically I could run them over much greater distances by daisy-chaining powered USB hubs and run lots of cameras simultaneously.
They also produce an IR intervalometer that operates cameras with IR triggering.
This hardware appears to be specifically created for the Stereo Data Maker (SDM) version of CHDK, however it works with the standard CHDK build. What’s more it works in the normal operation mode of the camera, not only in the “<ALT>” mode. For my own purposes I prefer the vanilla CHDK build although the SDM version has many alternative features.
Although the Gentles products are very good, and they produce a range of other stuff too, they do not meet all my requirements.