You can usually get away with light tethering on an unlimited data plan--forums, news sites, that sort of thing. Before I drowned my phone (have my SIM card in a GoPhone until I can get a break on something decent), I would sometimes plug it in if I felt like browsing on the train. They only get worried if your data use spikes into the multi-GB range.
I didn't even think about it at first--I foolishly assumed that my "unlimited data" plan entitled me to "unlimited data." Live and learn