Debian currently runs on the main webserver (cthulhu.gdc.wpi.edu - Dual Pentium Pro), and the main login server, which handles mail as well (lovecraft.gdc.wpi.edu - Alpha DS-10 - currently down due to hard drive failure). It is being considered for the three new servers that are planned for deployment in the next month.
I'm no longer the organization's sysadmin (I graduated) - however, I chose Debian for the systems I had control of because of my familiarity (using Debian since 0.96R2 - former maintainer of apsfilter and abuse), and because of the high security by default - as well as the application support, and hardware support for the Alpha.