I learned about this in passing during an urban planning class during undergrad (it was a great elective, and I really think more people should learn that stuff).
Combined sewer overflow [1] is essentially business as usual in many municipalities. I happened to learn about the Detroit sewer system, but even King Co in the PNW has this issue [2]. I like to think areas like that would think ahead about this, given the high annual rainfall, but separate systems are quite the expense.
http://www.mwra.com/03sewer/html/sewcso.htm