The iPhone is well-represented in the Northeast and Midwest, according to statistics released by Jumptap, a mobile ad network. The iPhone is also strong in Montana and Louisiana.
Android is very strong in the West, plus the rust belt of Ohio, Pennsylvania and New Jersey. BlackBerry remains strong around Washington, DC, with RIM being the provider of choice for government bureaucrats -- at least for now. Corporate-centric New York is also a stronghold for BlackBerry.
There doesn't seem to be any obvious correlation between iPhone or Android use and statistics like per capita income or population density. TechCrunch has the complete study.
The chart above isn't particularly informative -- it doesn't give precise market share breakdowns -- but it is an interesting look at which states are over representing one brand or the other.