Is there any point to giving the option for OSX? When was the last OSX version that did not support 64bit? Is there any reason to prefer a 32bit version on an OSX that supports 64bit?

Real questions, not rhetorical!

If I look at http://www.chromium.org/getting-involved/dev-channel , they list a Windows 32-bit channel and separate 64-bit channel, but only one Mac channel. I'm not sure if this means anything about their release intentions, or if the page may still be updated.

OS X has supported x86_64 applications since 10.5 (Leopard), so there's no problem there if your CPU supports it; moreover, the last OS X release to support 32-bit-only x86 CPUs was 10.6 (Snow Leopard). However, Chrome does support 10.6, so I guess this release may have the effect of dropping support for some Mac models from 2006 that previously worked.

It's customary to give the option when there's an option. If you go to the Chrome download page, there is no longer any option to download the 32-bit version.