If you ride a road bike, and the front caliper won't lock up at all, I'm guessing you either have worn-out pads, steel rims, or the brake cables need to be tightened.

I locked my front caliper the other day, when a pedestrian made an unexpected turn into my path. The rear lifted about a foot off the ground before I came to a complete stop. Fortunately the impact was minimal, and neither of us fell or were hurt. We both appologized profusely.

And as far as Sheldon's recommendation for only needing the front brake, I have to respectfully disagree. The rear brake is very useful for slowing down on long descents, additional stopping power on wet roads (rear-wheel skid is much more forgiving than front wheel skid), and using it reduces wear and tear on the front pads and rim. Fixed-gear bikes may not need them because pedaling slower should provide some rear wheel braking, but they do serve a purpose on a bike with a freewheel.