Currently, if you have unlimited data still on Verizon they officially say they may start throttling you on a busy tower after 2 GB. Most people who have actually been throttled by Verizon say that they used about 8-9 GB in a month before Verizon came over with an aluminum baseball bat to bash their data connection's speed in.
Verizon's throttling is interesting, as it doesn't occur all the time. If Verizon notices a heavy data user is on a tower which is congested, it will throttle them. If they are on a lightly-used tower at the moment, they'll still get full speed no matter how much data they are sucking down.
If you tether without paying for a tethering plan, all the carriers will ream your asshole if you are also a data hog. Except AT&T, they'll ream your asshole if they merely detect you tried to tether without paying. The other carriers will look the other way as long as you don't tether a lot and don't use too much data. Just so you know.