To put it in perspective. There have been multiple threads with people using multiple gigs of data on Virgin Mobile having their allowance moved down with no option for upgrade. As I cant see a scenario where you use that sort of data over 3G , we have to assume some were tethering. It IS expressly prohibited in the TOS for Virgin Mobile.
So basically - If you don't "extract the squirty stuff" (take the pee, whatever) your likely to go under the radar, when you get ON the radar the consequences are fairly severe. Your choice.