I feel for you. I have been through all this.
I have managed to get Sketchup Pro 6 (version 6.4.245) to work with OS X 10.3.9 Panther. On a Pismo of all things. It is running in evaluation mode at the moment, I will keep you posted on whether it reverts to 'free' mode at the end of the 8 hours. Mind, beware of all the links you can find on a google search. Honestly, I don't know why they make it so difficult, without an easy to find 10.3.9 download up front on the main site it is all too easy to be driven into the welcome arms of cr**kers phreakers spammers and goodness knows what else.
Here is what I did, and its all from the official source too (note health warning above):
Download PM101494ENA.dmg from the google website. This is the Sketchup 6 Pro version I am using. It is on the page 'Downloading older versions', not an easy page to find, here is the link
http://www.google.com/support/sketchup/bin/answer.py?hl=en&answer=60107
Click on the link Google Sketchup Pro 6 <Mac OS X> to get the file and save it to your desktop.
Once it has downloaded, double click on it to unpack it. Open the unpacked file. DO NOT CLICK ON THE INSTALLER. This will complain about wrong OS X version. INSTEAD, right click on it (ctrl click) and then click on 'Show Package Contents' from the menu which pops up. This will launch a Contents folder, which you can then open and then install the individual packages (there are 4) individually. All very demure, no mention of wrong OS version this way.
Voila, enjoy. Worked for me, all appears to function as it should, your own mileage may vary, good luck.