I arrived a bit late in the day, around 15.00. This is my first time going to FOSDEM, so I was not sure what to expect.
What I got was a lively bunch of free software and open source ‘enthusiasts’ (ok, hackers and geeks really), going in these lecture rooms. Tables on the corridors are populated by most well-known software projects, such various Linux distros, Mozilla, PostgreSQL, and others. The university settings of ULB gave it that extra touch of casual but studious atmosphere. I would guesstimate the attendace to be in the lower thousands.
As I am not due to present until tomorrow, I roam around a bit, and currently listening to the embedded power management session. As the whole place where FOSDEM is wired with wifi (meaning to provide wifi they run wires everywhere!), everyone is whipping out their laptops, smartphone, Nokia N810, Asus Eee and other devices, and probably doing three things at once.
All in all, if you are interested in free and open source software programming, this is the right place to be.