I've got to agree with Stuart, Daily Commute. Adding weight to the front wheel won't effect the point at which the rear wheel lifts off the ground. Let's think about a bicycle with an applied front brake as a third class lever. A third class lever is one where the force (the braking force trying to lift the rear wheel into the air) comes between the resistance (the weight over the rear wheel, keeping it on the ground) and the fulcrum (the hub of your front wheel).

An endo happens when the mass of the rider shifts toward the front wheel (which is the fulcrum on our third-class lever) under hard braking, reducing the force pushing the rear tire onto the ground. This force acts against the braking force, which is trying to swing the "lever" (your bike) up into the air. Shift it far forward enough, and there is no longer enough force on the rear tire to prevent it leaving the ground. Mountain bikers stick their butts back behind the seat when braking to increase the resisting force, which allows them, in turn, to brake harder. Luckily, because a bicycle acts like a third-class lever in this situation, the braking force must be greater than the resisting force in order to lift the rear tire.

The reason I'm describing this in terms of a lever is because force and mass at the fulcrum aren't relevant. Adding mass (panniers) at the front wheel, especially down low at the hub, simply does not effect the liklihood of doing an endo during hard braking. It will effect the stopping distance under a given amount of braking force, but added mass will have that effect no matter where it is on the bicycle. A bicycle with weight on the rear wheel is less likely to do an endo than a bike without weight on the rear wheel, but the bike with front panniers is no more likely to flip than an unloaded one. I hope this makes sense to you - loading up just the front panniers doesn't put you in any more danger of going OTB than riding with it empty.

EDIT: Chipcom, the point that I'm making is that weight on the front wheel has little relevance to balance in terms of weight distribution. Balanced weight between front and rear has relevance only terms of handling, and even then, on balance (HA!), more weight on the front is less likely to have a negative influence than more on the back. The critical factor for balancing mass in the front panniers is the right/left distribution of weight, which is indeed very important. For this, among other reasons, I just load up the rear rack, where distribution from side to side isn't critical, but in simple terms of fore/aft weight distribution, you're better off with 15 extra pounds in front than in back.