I know virtually no one uses Virgin Mobile, but many users (myself included) have speculated that they have implemented something in their network where if you enable wifi hot-spot capability on your device (previously allowed, but now only allowed with an added paid feature and only on two of their supported handsets), your phone would magically lose all ability to connect to VM's data network, and you had to do a factory reset to get data connectivity back.
Note--this was on vanilla handsets, not rooted/flashed. So while providers cannot block the use of 3rd party apps, no doubt they can still play games should they suspect you are tethering against some contractual agreement.
What i don't understand is so long as I'm using the allotted amount of data in a month as allowed by my contract, what does it matter to my provider how I'm using it? i.e., instead of writing emails on my crappy phone interface, why not open up a wifi hotspot and write emails on my (slightly better) tablet interface? Or my laptop? The data impact is essentially zero (okay, not exactly, but to the extent of practicality, it's zero). So who cares how I use the data? Oh, that's right, because providers are also trying to get you to buy their hotspot packages, external from your phone contracts...