According to this article, the NYSE is now running Linux on all it's core systems. This is really great news for a number of reasons.
Going back a bit, you will recall that Microsoft actually bankrolled part of the SCO litigation? The logic was that the Linux legal FUD would help them.
Alas, it seems to have gone a bit Pete for MS, with a tremendous backfire. Far from being driven into the arms of Microsoft, the NYSE has sided with the devil and decided that a non proprietarty unix like operating system is what they wanted.
NYSE have moved around 600 HP servers (yes, 600) to Linux.