The NY Times offers this editorial, which begins:

longstanding quirk in census rules counts incarcerated people as “residents” of the prisons where most are held for only a short time, instead of counting them in the towns and cities where they actually live. This practice was scarcely noticeable 30 years ago, when the prison population was insignificant. But with 1.4 million people in prison today, this padding of electoral districts’ population figures shifts political power from the densely populated urban areas where most inmates live to the less populated rural districts where prisons are often built.

UPDATE: See also this post on the ACSBlog.