I tested the force field map out on rcfreak0's server to see what it did with SMP. It's very different. For one thing, there's no force field effect on players, only on other carts.
Secondly, if you break any of the glass around the cage, carts will start floating out and slowly drift towards the ground. In SSP, you can't place blocks anywhere in the area without being sucked into a minecart. That tells me that what's happening is a disparity between the location of the cart in the physics engine and the rendering location. The physics engine appears to think the carts are forced beyond the walls and the cart pressure is what pushes you back.
