For the record, I plugged in our old G3 laptop (333Mhz) running MacOSX 10.3.9, downloaded arduino-0011 mac, installed the ftdi drivers it came with, and connected my solarbotics Freeduino. The IDE came up fine, and it successfully compiled and uploaded blink.pde, which ran fine on the freeduino. So I think it should be possible to run on a G3 iMac as well.
The 10.3 os did want me to install a bunch of updates, including java updates, and I did these before trying to run the IDE.
It was rather painfully slow. Sigh. I remember when a 300MHz CPU was
pretty zippy...
I haven't tried Arduino-0012 yet. Perhaps tomorrow.