It may not be the case in a few years time (
http://www.theregister.co.uk/2005/03/03/pc_tax/ ) but yeah
http://www.tvlicensing.co.uk/information/index.jsp
"You need a TV Licence to use any television receiving equipment such as a TV set, set-top boxes, video or DVD recorders, computers or mobile phones to watch or record TV programmes as they are being shown on TV.". (I assume as it is being shown = live which could possibly drop you in it for stuff like live football and I would say drop the TV capture card if you have one).
I find it hard to see how usenet and DVD rentals count towards that let alone a VGA box.
You will still have to deal with the 400,000 threatening letters they send as it practically unthinkable to not have a TV apparently (I got multiple letters a month when I did not have a TV 2 years ago, of course I did not give any details which probably did not help).
I think it is probably safe to call bull shirt on the black vans/helicopter theory (I find it quite hard to reason with physics how such a thing would work (TVs receive/decode only so there should be no transmission).
As I understand it most people are caught because they have one on display through a window which then leads to warrant/court order or they are foolish enough to allow inspectors in.
This site may prove of some use as well:
http://www.marmalade.net/lime/
*turns on Faraday cage.
Also I seem to recall Scorpei reviewed one of these boxes a few months back.