The HardwareSecrets gang has put up an overview of just how it's possible to push 1Gbps through cabling only meant to sustain 100Mbps. Good stuff.

"Ethernet standard uses a technique against electromagnetic noise called cancellation. As electrical current is applied to a wire, it generates an electromagnetic field around the wire. If this field is strong enough, it can create electrical interference on the wires right next to it, corrupting the data that were being transmitted there. This problem is called crosstalk."