The Advertising Standards Authority have looked into this in the UK, and here’s their position:
“Something can be described as “unlimited” even if a fair-use policy exists. However, the existence of the fair-use policy should be stated in the ad, and the policy should only be invoked to prevent misuse of the service.”
http://www.asa.org.uk/asa/focus/background_briefings/Telephone+and+broadband+packages.htm
This is just a licence for companies to give whatever limits they like but still say things are “unlimited”, and makes me wonder what the point is of having an Advertising Standards Authority in the first place. The Authority’s only power seems to be to punish companies who leave off the small print specifying the fair use in their adverts.
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