An interesting post by Darren Waters, editor of the BBC News website's technology index. He has been monitoring his experience of broadband speeds on his Virgin Media "super highspeed" broadband service. For most applications the actual speed (ie how big the file is divided by how long it takes to arrive) is a lot lower than the headline speed of 50Mb/sec.
The blog shows that the advertised headline speed is fairly irrelevant to the actual experience of an average customer, even when the provider can actually deliver such speeds reliably within the confines of their own network.
The limitations, especially in the newly designed cable broadband networks, are not at the access layer of the network (the wire from the local network node to the home). They are out in the internet world beyond.
It just goes to show that messages about headline speeds can only disappoint. Maybe it's time for Virgin Media to change their marketing, rather than fiddling further with their technology.