I've had a Virgin Mobile SIM for over 2 years now and I've never been charged for tethering. Alright, I haven't tethered much before, but I still haven't been charged for it.
This is on the 3.5GB data plan that was sold as Unlimited (though it isn't unlimited, due to the steep slowdown @ 3.5GB).
Last month, I did get reasonably close though - 2.35GB total; 1.83GB tethered
The previous month (Feb - Mar): 1.25GB total; 0.93GB tethered
So far this month: 0.89GB total; 662MB tethered
Most of the tethering is due to our cranky VM Superhub playing up, my laptop says "Resolving host" and refuses to load anything - so out comes the hotspot.
In all honesty I really don't like Virgin Mobile, due to:
1) No 4G
2) No official tethering support (you can tether though)
3) Mis-selling unlimited plans with FUP*
*I heard one story on the Virgin Media forums whereby a user was complaining about poor data speeds. Their speeds were initially fast, but then went very slow. They tried a whole array of troubleshooting techniques, including (but not limited to): different locations, trying the SIM in another phone (still slow), trying a different SIM in the phone (separate SIM was fast) and factory resetting the phone (did nothing). They were advised that there was no speed limit. This person had very little access to Wi-fi, hence paying for the unlimited package. They were then told (by a different user) to check their usage, as it sounded like they had hit the hidden 3.5GB usage cap. Usage check showed them as using 8GB data. Ouch!
Although I cannot remember exactly what happened after that, I think the user left and went to Three, seeing as Three don't have a FUP like that (they do have TrafficSense though).
I'm not worried about Virgin Mobile anymore either. I've taken the 16GB EE SIMO deal that was on promotion a few days ago. So I'll be leaving VM Mobile as well.