The problem you are all experiencing is an intermittent connection of the filament in the bulb. It's a common problem in old light bulbs. The filament is broken, yet the two broken ends are still touching eachother. As the bulb warms up, the metal gets softer and they separate (only by a little bit). Tapping the bulb hooks them back together, but it's only a temporary fix as I'm sure you have all noticed. A new bulb will fix the problem.

EDIT: 18secFerio, who is that in your avatar? Hot!