They starting blocking tethering on my account earlier this year. It's been working fine for the past 3 years but according to Virgin's support team they "don't allow tethering". Unfortunately I can't find my 3 year old contract to see what it says about tethering.
Looks like I'll have to jump ship when my contract comes up for renewal. The annoying thing is that I only tether once every few months and NEVER use much data, in fact my plan is capped at 1GB.