Why are people in Iowa interested in combinatorics? Combinatorics is more popular in Iowa than in any state but Massachusetts.
Google now has a feature called "Google Insights"; you can type in a search term and see where people are searching for it, how frequency of searches varies with time, etc. In states where there's a lot of volume it's possible to zoom in; in Massachusetts it's possible, for example, and most of the interest is in Cambridge. Given that there is a Big Important University and a liberal arts school that has a well-known mathematics department in Cambridge, that's not surprising. But I can't zoom in on Iowa.
(It's possible to get results by country, too, but these results seem ridiculously skewed; I suspect that Google may be normalizing by the number of Internet users in a given area, and the user pool is different in different places.)
Another interesting one: "probability" is popular in Maryland, and among cities in that state it's most popular in College Park and Laurel. College Park is where the University of Maryland is. Laurel is where the NSA is. You can see similar things in other states; for example, in New York, "probability" is most common in Stony Brook, Troy (RPI), and Ithaca (Cornell). In Pennsylvania, it's University Park (Penn State), Bethlehem (Lehigh), and State College (Penn State again). The general pattern seems to be first a few college towns, then the big cities -- the places with the fourth and fifth highest numbers for "probability" in Pennsylvania are Pittsburgh and Philadelphia.
Most mathematical search terms I could think of are highly seasonal -- they're less common in the (Northern Hemisphere) summer, when schools aren't in session. That seems to imply that lots of the people doing the searching are students. I couldn't find a mathematics-related search term that didn't show this seasonality; I don't know if it can be done, because only search terms that receive a reasonably large amount of traffic are reported on the site at all, and things which are important enough to get lots of traffic are probably studied in schools.