well 5GB has been the standard definition for fair use in the mobile data industry for the past few years. THE ONLY REASON that this invisible cap exists on supposed "unlimited" plans is because it is the ONLY way they can tell whether or not you are tethering (using your phones data connection for your computer) using non approved methods. Computers tend to use significantly more amount of data than a cell phone. So, if your cell phone was pulling in excess of 20 GB per month, compared to the average of say 700 MB, then there was a problem. You were then considered to be "abusing" the "unlimited" data access.
Nowadays, phones can stream audio and video content and can download files in excess of 100MB in as little as 15 minutes. This is why AT&T has moved to a tiered data plan structure, swaying away from the old trend of "unlimited, but with Conditions" model. Verizon is soon going to follow suit with a tiered system as well, so that both companies can maximize their profits based off of individual data usage, which sucks!
The only company that hasn't bulgded on this issue as of yet, is Sprint. They, too have an unlimited fair use policy (ususally at 5 GB / month), but with the release of the Evo, Sprint went in the opposite direction of its peers and charged an Extra $10/ month for the Evo's 4G capabilities and gave what it calls "true unlimited" data, which means there is no fair usage cap. So, I can stream all the pandora and YouTube I want. Also, their plans are the cheapest.
That being said, the latest rounds of smartphones are more likely to break that 5 GB cap (regardless of what AT&T will have you believe about their iPhone customers) because of its data capabilities. The latests reports out show that Android phones on verizon is pulling more data than the iPhone on AT&T. Streaming audio and video on your phone is a sure fire way to run up your data usage. This is done way more so than browsing or widgets using data. SO, if you spend your time doing a lot of streaming, thats whats sucking up your data.