In philosophy in the US, graduates of the top 5 programs got 37% of the total jobs. Graduates of the roughly 75 programs not ranked in the top 50 all together got 12% of the total jobs. The impact of prestige on hiring is overwhelming.
Spurred by xLeitex's comments below, I did a bit more research and thus am
editing my answer to provide a few more links I've found, since I think the topic is both important, and it comes up somewhat regularly on the site.
First, Baldi 1995 found that "job placement in sociology values academic origins over performance."
Second, Burris 2004, 250 argues that the "social capital" involved in coming from a prestigious department affects not only one's placement into a first job, but also one's subsequent academic career.
Third, Long 1978, 902 argues that scholarly productivity is "facilitated by department location" but that "productivity, as indicated by measures of publication and citation, plays an insignificant role in the selection process."
Certainly the picture that these three articles present makes it looks like prestige of one's PhD granting department isn't merely correlated with getting a successful academic career, but is an important causal factor.