I'm curious as to your source or why you say, in no uncertain terms, that Houston is "the most diverse in the country". If such a thing could be accurately quantified, I would bet my life savings on NYC.
And the relevant paragraph regarding NYC: The standardized Entropy Index measures how close a total population is to sharing balanced percentages across its racial/ethnic groups. If one subgroup is 100 percent of the population, then the standardized index score will be 0. If the four racial/ethnic groups are each 25 percent of the population the score would be one. Houston has the highest entropy score of the 10 largest metropolitan areas, 0.874. New York is a close second with a score of 0.872. All but two of the 10 largest metropolitan areas, Boston and Philadelphia, have a higher entropy score than the national average of 0.709, meaning that in general the large metropolitan areas are more diverse than the nation as a whole.
EDIT: I'm not going to argue the pros or cons of the methodology because I'm not familiar with the field of study.
I am not an expert in its use, but IMHO, it's BS because it's dependent on the idea that you have maximum diversity if various subgroups are all the same size, but it's very dependent on the definition of subgroups. For example, in the case of ethnic diversity, the measure will vary significantly depending on whether you group people from Southeast Asia and people from the Indian subcontinent together as "Asian" or not, or any other subjective subdivision.
It might have use as a comparative measure with a predetermined set of classification groups, but you can definitely manipulate your conditions to construct different outcomes.
http://en.wikipedia.org/wiki/Demographics_of_New_York_City
I would go with NYC also, but its not too far off.