I went through this with T-mobile. I bought a G1 back in 2008 and kept that data plan even after my contract expired. This was an unlimited plan with throttling after 5GB.
The original terms of the plan said nothing about tethering. This means that it niether allowed it nor disallowed it. I started buying nexus phones and regularly using the built in tethering. I used it when I moved (my wireless router at home can use the phone as its internet gateway) and then almost everyday at work to get around our filters.
Eventually, T-mobile noticed that my usage was hitting (or getting very close to) 5GB/month and they started blocking my tethering. I called in and spoke to some customer service folks and eventually reached an agreement.
In my case I had been a customer for many years, was no longer under contract and was not violating any of the terms of my original agreement, so all the changes made were administrative on their end. I got to keep my same level of service at the same price point.
A few things: Your phone was not made to be a full-time router. It will heat up if you pull a lot of data through it and the heat cycles will eventually break something. I know this because it happened to my wife's old Galaxy Nexus which we had been using as a wireless Netflix box for a few months on our plasma TV (via an MHL adapter). One day the phone just stopped sending a picture to the TV, even though everything else worked (probably busted an internal capacitor or something).
You might want to consider a service like CLEAR, or a wireless repeater and sharing wireless with a neighbor. Failing that, get a back-up phone so that you don't lose all communications if something does go wrong.
