Pittsburgh doesn't have to become as unaffordable as NYC, DC, Boston, etc., but to avoid that fate it needs to be developing new housing units fast enough to keep up with demand, and ultimately it needs to be investing in things like rapid transit expansions as well.
To this limited extent, the model we should really be looking at when it comes to housing development is the Texas cities, places like Houston and Dallas, which have managed to retain affordable housing despite rapid population growth. And their solution is not magic--it is just really easy to build new units there.
However, high transportation costs in those cities offset a lot of their housing affordability (as do lower incomes, which is a whole other story). And low transportation costs is where cities like NYC, DC, and Boston shine (and also on the income side).
So, if we could combine Texas-like housing policies, with Northeast Coast-like transportation policies, with the already solid income growth we are seeing--that could keep Pittsburgh out in front when it comes to overall affordability indefinitely.
Edit: There are a lot of great statistics and some good policy recommendations in this (relatively) short report:
http://www.nhc.org/media/files/LosingGround_10_2012.pdf