SELinux was a factor in the recently announced NYSE Euronext adoption of Linux:

NYSE Euronext also relies on Red Hat Enterprise Linux and its included Security-Enhanced Linux (SELinux) functionality to preserve the security of its platform. With trillions of dollars flowing through the exchange each month, security is a large and very important organizational focus. “We are very security conscious because we have to be. The operating system is a key part of every server that we operate and each server must be secure at all times. We maintain the security of our systems by relying on the SELinux features within Red Hat Enterprise Linux,” said Rubinow.

Their system handles US$141 billion per day in trades.