Same problem here... the two UNOs That I got work just fine on my Intel IMac, but no go on my Titanium G4 as well... the only two leads I've gotten so far are:
!. 10.5 might
fix the issue... I gambled and ordered a gig of RAM for about $60.00 so I can upgrade the OS
2. A vague reference on Arduino playground that states "Minor detail: on my OS X 10.4 box, I had to add a "-C /location/to/avrdude.conf" to AVRDUDE_FLAGS otherwise it's bombing out when avrdude wants to upload your code." --cramur
However, I have no idea where to insert the line... if any of you know where AVRDUDE_FLAGS is and where I can find my avrdude.conf file (appears to be a hidden file) then maybe we can all solve this together!