Ahh... there it is. Thanks Susan & Jeff (& everyone else).
I'd tried this off a hunch from posts I'd seen on another issue, but since SU gave showed a message it would take effect for new windows I'd simply re-opened the file (rendering looked different, but still showed the issue). Now that you've confirmed, I changed the setting and then quit & relaunched SU and indeed the issue disappears.
Also sadly indeed, it is barely useable in that mode. Guess I can do most of my building/manipulating with the acceleration on, then turn it off for establishing some final views with cuts. Was hoping planes could be more useful for me than that, but maybe with my next computer (since I don't dare imagine Google addressing this older, apparently rather specific hardware issue in future releases).
Thanks again everyone. Incredible resource we've got here.
--Ben
p.s. if anyone is interested in the details, I'm running OS 10.4.11 and Sketchup for Mac 6.4.120 on a 1.42GHZ iBook G4 which the spec says has a 4X AGP ATI Mobility Radeon 9550 with 32 MB of DDR SDRAM