@GreyLinux bru airport is 30 mins away by train (except Ryanair who fly to Charleroi, watch that), trains v frequent.
ULB is a bit out in the sticks, so hotel doesn't matter too much. Stay anywhere in the centre (we use the Novotel) and follow the crowds to the busstop around 9am - 7000 people go to FOSDEM, so it's hard to miss ;)