Check out my response in the "unlimited" thread. People should speak with billing and BB tech support and not customer service who are the front line and do not know much about billing/data. As explained, with the upcoming release of the 8320, WiFi and HSDPA, Rogers wants to offer unlimited but also needs to safeguard from resource abuse.
They offer unlimited so people will not buy an 8320, jump the Rogers bandwagon and go 100% WiFi. With HSDPA, this spectrum costs a fair amount of money and with speeds similar to ADSL (I say similar since they still throttle them), I can easily see people cancelling their home service (Telus/Shaw) and tethering the device. Although possible, this is not meant to be the purpose of a mobile device but I can see people doing this to save money. These are the people who when a pattern is detected, they will charge $7MB over usage.
Rogers reserves the right to charge this but you must understand, they will not blatantly charge but will notify you first stating based on account review, you are abusing the system. They will ask to to cut back and if not, you will be charged. If you continue, they will implement a charge.
As for how much data you use, you can always log in to your BB account from your home computer and check your data usage. If not real time, it's updated within the last 24 hours.
Just like the CityFido plan and Unlimited FIDO GPRS, there was a cap. CityFido could not exceed 5,000 minutes and the unlimited GPRS could not exceed 250MB, if so, they stated they reserve the right to charge. Has this ever happened? Not really. There was one person who it happened to but their CityFido line was something like 10K - 12K minutes per month and this was just utter abuse. They did not have to pay but after multiple warnings, they were told if it continued, they would not be able to renew the plan when up for renewal and would have to chose a different package.