Even with "unlimited" cellphone plans, there's a limit: the number of minutes in a given month sets the limit. However, cellphone companies use the "unlimited" word because consumers go ga-ga over it; and because listng "10800 minutes' use cap per 30-day period" seems kind of silly. Cellphones don't track bandwidth or disk space usage; they track the number of minutes a given service is used. Thus, they're getting away with semantic trickery. (I would LOVE to see someone use their cellphone, with its unlimited plan, for every minute of a given month for several months running and see if their provider doesn't try to shut them down.)
You can say you've got unlimited hosting today. Plenty of hosts do, because they know that most users won't use anywhere near the gigantic amounts that people seem to look for. But the first time that an "unlimited" web host shuts down a client because that client has used an inordinate amount of disk space or bandwidth, they've just lied to their entire client base: they said "unlimited" but the first time someone actually tries to use a petabyte of disk space and three terabytes of bandwidth, they're likely to be shut down if they're on one of the cheaper "unlimited" hosting plans.
Now if a web host charges $200 a month for a truly unlimited shared web hosting plan (ie, someone could actually really use close to a petabyte of disk storage space and several terabytes of bandwidth in a given month) and has the capacity to loadbalance so that their other clients would not be adversely affected when people do actually use a gargantuan amount of resources...well, then, those hosts might have the right to call such plans "unlimited". Thing is, not too many people will pay $200 a month for any shared web hosting plan. A managed server? Yeah, sure - even more than $200, depending on server specs. But pay $200 a month for a shared hosting plan, even one that was truly unlimited? I don't know how many consumers would go for that.