Media Matters for America has studied it. The results are as expected (white and male is the main flavor in the diversity soup) but also interesting, because of the differences in different programs.
This study is better than the earlier ones because it gives population percentages as the basis for comparisons. As I've written many times before, I don't like the term "diversity" that much because diversity can be achieved by taking a gallon of white soup and adding a quarter-teaspoon of blackness into it. Or similarly for gender and ethnicity. In short, it's fairness, really, that I'm interested in, not specks of other colors or genders in the sea of one color or gender.
Here is one picture from the study:
Now, the question of fairness is one of those long-term goals: a society where the positions of power are roughly reflective of the various demographics. But in the short-run those news programs must also take into account the fact that white men are a higher percentage of the powerful than of the general population, and any realistic political program must reflect that if it wishes to interview politicians in the US Congress, for example.
On the other hand, it is possible to further influence this trend, and one way of doing that is to pick topics which are of greater interest to one group of viewers than the rest, to invite guests from mostly that group, and to consciously or unconsciously omit other possible guests. As an extreme example, there have been political shows where only men discuss abortion questions, and my guess is that we might have a program where only non-Latinos are present to discuss immigration questions. And so on.
The question of which topics are deemed important is closely linked to this question of representation. I think the links are complicated, however. Certain issues are labeled as "women's issues," for instance, and then women are invited on the panels. That's both bad and good, I guess. Good, because at least those issues get covered, bad, because the way they are covered exacerbates the idea that issues which really are everybody's issues (workplace flexibility, say) aren't of general interest. Then the "general interest" issues are covered without thinking that women have anything much to say about them. After all, they got the girls' segment last week.
That's a muddle, but I hope you get what I mean. All these concerns are intertwined and to some extent rise from the same basic problems.
On a more superficial level, being white and male is the unmarked option. Belonging to any other group then becomes the marked option. Thus, to be viewed as an individual rather than as a representative of a group is easier for whites and/or men. But paradoxically, to get to the point where everybody is viewed only as individual merits requires that all groups have roughly the same representation. That way a particular guest stands for only his or her own opinions and not for "what women think" or what "people of color think."