The new site design1 has as part of the background image this:
It appears just to the left of Questions, Tags, Users, etc., at least for me. It appears to be a hand-written case-number or something on some kind of old official form – which seems quite fitting, but now I’m wondering if that number itself has any significance. A Google search didn’t return anything.
1. And quite possibly the old one, since I’m a lurker and didn’t spend enough time here to say for sure it didn’t have it... but I’m pretty sure it’s new.