If someone advertises "unlimited data" I expect unlimited data. The way they usually avoid abuse is by capping high-speed (3G or more) transfer to only some amount (e.g. 10GB/month), after that you get a "slow" connection... but even that should be clearly stated in the advertisement.
Here in the UK Ofcom (governing body over communications) are pretty strict with what you can adversise. For home broadband, providers have to give realisic numbers to prospective customers - not just an "up-to". For mobile, they have to be transparent in what they provide. So unlimited data, is just that, unlimited. End of last year there was an investigation opened against two providers due to throttling and limiting tethering data (domestically, still have roaming restricitions) when under an unlimited plan. They both agreed to remove the limits as they would both likely have to change their advertised plans if they didnt.
