Refer to Google Chrome accessibility update
Chrome Frame 14 accessibility support
As I said this a quick revisit, so will get straight to the point. It is my understanding that Chrome Frame development tracks the chrome browser, including accessibility support. Chrome and hence Chrome Frame does not support labelling of form controls using standard HTML methods.
Note: older versions of Internet Explorer and current versions of Safari only support the for/id method.
The chrome team are working on accessibility issues. There are still quite a few other issues such as lack of support for Windows High Contrast mode. I believe the lack of support for form control labelling alone is a showstopper. The chrome team are aware of the issue, lets hope it gets fixed soon. Check out HTML5Accessibility.com for browser accessibility support issues with new HTML5 features.
So while Chrome/Chrome Frame is no longer an accessibility black hole, it is still akin to looking through a glass darkly, for some.
Great to read about accessibility improvements in Chrome 15!