The limitation on USB cable length has to do with the USB standards for signal timing and the type of cable used. This basically means that the longest cable length you can have between a hub and a device is 5 metres.
However, as the hub regenerates the signal timing, you can daisy chain hubs to a maximum depth of 5 devices, or 25 metres.
The "active usb cable" mentioned before basically contains a single port hub in the end, so it provides the electronics to regenerate the signal.
There are also USB-to-fibre devices that will allow you to work USB over a much longer distance with fibre optic cable.
Andrew