The HDcctv Alliance, a group that has been espousing the virtues of running HD video over existing coaxial cable, has now announced a version to run over Cat5e and 6 cabling.
One rational cited was to allow "Legacy VGA IP camera-based systems can easily upgrade to uncompressed HD with HDcctv CX." All you need to do is drop your network switches, get cameras from a new supplier and add in a HDcctv DVR (or encoder).
Let me know if you are interested in this.