Sorry but this absolutely isn't true. After seeing them so highly recommended on this forum recently, I grabbed a Virgin 'big data' tariff SIM this week after buying my GS3 LTE. I then duly topped up £10 for 'unlimited' data with tethering. After syncing my new GS3 LTE and my tablet (flashing ROMs, re-downloading apps, backups etc) I hit 10GB of downloads quite quickly (inside a few days).
Strangely, as soon as I hit the 10GB mark my speeds hit a perfect 0.3Mbps no matter where (or at what time, or on what server) I tested. I phoned customer services and the guy tried to tell me it was due to utilisation in the area for which they are performing tower updates this month, and "it will reset at the end of this month"...
So, I said, you're basically blagging that I've been capped/throttled for hitting 10GB inside a month and the throttle will reset when I top up next month?... He denied this at first so I pushed, and asked what upgrades were they undertaking, and did that mean I wouldn't slow down after grabbing 10GB + next month after the 'upgrade'?
He started stuttering and eventually said, look, you've hit our FUP of 10GB and your browsing speeds will reset at the end of your top-up month. If you stay under 10GB next month you won't slow down, but if you go over you'll slow to 0.3Mbps or less again for the rest of that month, too.
Unlimited my arse.

I jumped to Three on a pay-monthly SIM today (One Plan, my second contract with them) and am now downloading and surfing at 22 to 25Mbps no problems.
It's a shame because T-Mobile's network actually seems quite decent. I get more signal than I do on Three (marginally) and in more places. But Virgin? No thanks. Maybe it's worth me jumping to the full monty on pay monthly, but I'll probably just stick with Three now.
Edit: I'm not the only one it seems.