On Some Random Webpage Full Of Spam, I came across what purports to be US News' 2009 ranking of graduate programs in mathematics. (I feel bad about linking to this, because it just helps the spammers, but I'm doing it anyway.)
If I remember correctly, this ranking is produced by surveying people in mathematics departments at various schools and asking them to rank other institutions. That's it.
It seems to me that a more sensible way of ranking mathematics departments would be to start with the assumption that a better department is, by definition, one which has its students get hired by better departments. This could work, at least to do the high end of the ranking, because the departments doing the hiring are often the same departments that have graduate students; eventually I expect the process I'm alluding to would converge on a ranking. I'm not sure what you'd do to deal with ranking "lesser" programs where many students are not hired by departments which themselves have doctoral programs. This just aggregates what people think about reputation, but in a way that's more principled than just asking some shadowy panel thinks.
Of course, in the end these sorts of rankings are not particularly valuable, so I don't want to pursue this any further.
(Disclaimer: my memory of how these rankings work comes from standing in a bookstore four years ago and copying down that year's rankings on a scrap of paper. I think the statue of limitations has passed on that, so I'll admit to it.)