Bump with a new bit of info:
http://www.thinkbroadband.com/news/i/3760.html
Thinkbroadband appear to disagree with Virgin Media's assertion.
I also note that other speed testers appear more than capable of handling 50Mbit as well.
http://www.speedtest.bbmax.co.uk - confirmed, measured via 100Mbit result ~90Mbit, uses a customised Ookla tester.
I do partly agree with VM, however speed testers that perform badly are bad speed tests, nothing more. Good ones such as the above happily cope with 100Mbit+
http://members.chello.nl/m.bhola77/120.jpg
Even much maligned speedtest.net is ok, so long as you use the right servers:
http://www.speedtest.net/result/337150440.png
A very poor speed tester that consistently gives bad results on anything apart from low bandwidth links and should be avoided is, amusingly, this one:
http://www.broadbandchoices.co.uk/speed-test.asp
For all their metrics, etc, I have never been able to break around 6Mbit on their tester across two ISPs, while others give a good result. The reason for this is simple. Thinkbroadband and BBMax live off n x GigE kit in Docklands, while this tester lives in a host-it.co.uk datacentre in either Milton Keynes or Northampton and appears to have 100Mbit maximum. Their off the shelf unmetered bandwidth goes up to 50Mbit/s which costs £1050 a month.
Seems a bit strange that various other places in the world that have had 50+Mbit for a while haven't suddenly gone up in arms over speed tests, while the UK, home to the 2nd largest by volume Internet Exchange in the world, suddenly is going to have issues with 50Mbit.