Historic Pittsburgh is working on their own census project that will allow users to search for ancestors in the 1850-1880 US census for the city of Pittsburgh and Allegheny City for census years 1850-1870. They also have boundaries maps available for viewing. It may be helpful to read over the Census help link on the Census Schedules home page. This is a worthwhile project for researchers with Pittsburgh roots.
While you are there, check out the many other digital archives they have to offer, such as photos, maps and videos!