This could be a possible way to get an idea.
Notice I said "possible". :)
Since I highly doubt that there is any info that is accurate to be found, except it if came from Yahoo. Firm data would be sought after and paid for by most, I am sure.
Looking at the top 500 or 1000 search terms that are usually up there month after month can give you a feel as to which categories are called upon the most at Yahoo.
You know that Yahoo has more searches done daily than any other. You also can figure out "groups" or "categories" that the top search terms may be in.
mp3, music, games, toys, "...", etc, are all highly searched on.
You can search for the top phrases in Yahoo and see which categories come up first, 2nd, etc.
I know that SEO search terms are not that popular with Yahoo searchers,...or not as popular as lots of other terms.
This way is very primitive, I know, but without actually knowing the data, finding the absolute most popular categories is almost impossible.