Before! You shouldn't work out on a full stomach. I suppose it depends on your routine, and what you're eating - I can quite happily do an hour of Body Combat on an empty stomach provided I've had a decent dinner the night before and I've got Powerade to drink. However,I'll sometimes get up extra early and have something liquid at least an hour beforehand, which is better really, but it does mean getting up very early sometimes.
It also depends what exercise you're doing. If it's fairly light stuff, don't worry too much. If it's strenuous, you need to have eaten, but most definitely not right before the workout.