they seem to be testing on science, education and medical terms, from what I've been able to find. Headings are not the same as the 'related searches' at the bottom of the page, so its hard to see exactly what criteria might be being used, and since they're not on especially commercial SERPs (yet) its also pretty hard to second guess how broad match the ads may be setup.
If anyone finds a comment on this elsewhere could you please post a link? Cheers.
