I don't why you'd have to be any more vigilant than you'd normally be. If the pedestrian is in the street while you're driving, you should see him anyway. I'm sure hybrid cars still have horns, right?
Roughly half the time a pedestrian gets hit it is their fault, half the driver's. You still have control over your half, it's just that they can't hear you coming so sometimes they will step right out in front of you and there's nothing you can do to avoid the collision. Another problem is you can no longer run a yellow light in the city. With a conventional car, the pedestrian hears the car racing. In a hybrid they take their walk light right away and get hit. A hybrid driver does not have the high sitting visibility that an SUV driver has, so extra care is needed. Personally I wouldn't city drive in silent electric mode without buying a lot of ambulance chaser insurance.