http://pa.speedtest.rcn.net/newtest/ [Mod edit: The linked test is for rcn network users and thus will not work on Virgin Media]
Wouldn't it be great if VM would give us some stuff like this instead of 'download 2 files from ftp.virginmedia.com and add them up...'
Click the advanced view and you can see where it would populate fields showing more information on the cable modem.
Of course for Virgin to supply this it would mean some work on customer facing tools and we know how allergic to that VM have been so far.