Bullshit cellphones have unlimited internet. It may have to do with your provider but listen to this. I have had multiple prepaid no contract phones as well as a few contract services who say it is unlimited internet but it really is not. After you hit their magic number usually about 2 weeks into the month you will experience SLOW connections, so slow and so consistent that it is obvious they are throttling your speeds and in some cases just shut you off until the next billing cycle.
As far as internet providers I have seen a couple where it is written in the contract that there is a limit but that they are not currently holding anyone to it. Comcast is by far the worst when it comes to this on top of their shitty customer service and intermittent connection. Its no wonder they have to have a monopoly in any given area because I don't think anyone would chose them if they had another option with similar speeds.
http://customer.comcast.com/help-and-support/internet/common-questions-excessive-use/I hate everything about Comcast they are an evil monopoly. I refuse to give them any business. I sure do miss having fast speeds but with the connectivity issues they always have and then they cut your speeds down to a crawl its not worth the 50 dollars a month. LoL 50 who am I kidding, there is always another 20-30 in hidden fees on top of it all.
I currently have DSL. Sure its slow but it NEVER goes out. Thankfully the phone company in my area is totally rebuilding their network and will be offering HDTV, DSL internet with up to 25meg "about half of cable but its a start" and of course phone.